THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


REFRACTION 


HOW  TO  REFRACT 


THORINGTON 


BY  THE  SAME  AUTHOR. 

Rctinoscopy  (The  Shadow  Test)  in  the  Determination 
of  Refraction  at  One  Meter  Distance  with  the  Plane 
Mirror.  54  Illustrations,  a  number  of  which  are  in 
Colors.  Fifth  Edition.  Cloth,  net,  $1.00 

From  The  Medical  Record,  New  York. 

"  It  presents  a  clear,  terse,  and  thorough  exposition  of  an 
objective  method  of  determining  refraction  errors  which  is  de- 
servedly increasing  in  popularity.  In  our  opinion  the  author  is 
amply  justified  in  declaring  that  its  great  value  in  nystagmus,  young 
children,  amblyopia,  aphakia,  and  in  examining  illiterates  and  the 
feeble  minded,  cannot  be  overestimated,  and  we  agree  with  him  in 
reminding  those  who  attempt  retinoscopy,  fail,  and  ridicule  it,  that 
the  fault  is  behind  and  not  in  front  of  the  mirror.  The  book  is  well 
printed  and  usefully  illustrated." 

The  Ophthalmoscope  and  How  To  Use  It.  With  De- 
scriptions and  Treatment  of  the  Principal  Diseases  of 
the  Fundus.  With  12  Colored  Plates  and  73  other 
Illustrations.  Cloth,  net,  $2.50 


HORIZONTAL  SECTION  OF  THB  RIGHT  ~E\K.—(Landois.) 

Cornea,  b.  Conjunctiva,  c.  Sclerotic,  d.  Anterior  chamber  containing  the 
aqueous  humor.  <?.  Iris.  //',  Pupil,  g.  Posterior  chamber.  /.  Petit's  canal. 
j.  Ciliary  muscle,  k.  Corneoscleral  limit,  i.  Canal  of  Schlemm.  m.  Choroid. 
«.  Retina,  o.  Vitreous  humor.  No.  Optic  nerve,  q.  Nerve-sheaths,  p.  Nerve- 
fibers.  Ic.  Lam  ilia  cribrosa.  h.  Crystalline  lens.  or.  Ora  serrata.  pc.  Ciliary 
processes.  The  line,  O  A,  indicates  the  optic  axis  ;  S  r,  the  axis  of  vision  ;  »-,  the 
position  of  the  fovea  centralis.  A'n.  Nodal  point,  x.  Equator  of  lens.  /.  Ex- 
ternal rectus  muscle,  s.  Internal  rectus  muscle.  Z.  Optic  nerve-sheath.  //. 
Sclerotic. 


Ml'SCLES   OF   THE    EYE.      TENDON   OR    LlGAMKXT   OF   ZlNN. 

I.  Tendon  of  Zinn.  2.  External  rectus  divided.  3.  Internal  rectus.  4.  Inferior 
rectus.  5.  Superior  rectus.  6.  Superior  oblique.  7.  Pulley  for  superior  oblique. 
I.  Inferior  oblique.  9.  Levator  palpebrae  superioris.  10,  10.  Its  anterior  expan- 
sion, it.  Optic  nerve. 


REFRACTION 


AND 


HOW  TO  REFRACT 

INCLUDING  SECTIONS  ON  OPTICS,  RETINOSCOPY,  THE 
FITTING  OF  SPECTACLES  AND  EYE-GLASSES,  ETC. 


BY- 
JAMES  THORINGTON,  A.M.,  M.D., 

PROFESSOR  OH  DISEASES  OF  THE  EYK  IN  THE  PHILADELPHIA   POLYCLINIC  AND  COLLBGB 

FOR  GRADUATES  IN  MEDICINE;    MEMBER  OF  THE  AMERICAN  OPHTHAL- 

MOLOGICAL  SOCIETY;    FELLOW  OP  THE  COLLBGB   OF 

PHYSICIANS  OF  PHILADELPHIA,  ETC. 


UbirO 


TWO  HUNDRED  AND  FIFTEEN  ILLUSTRATIONS 

THIRTEEN    OF    WHICH    ARE    COLORED 


PHILADELPHIA 

P.    BLAKISTON'S   SON   &   CO, 

IOI2     WALNUT    STREET 
1907 


COPYRIGHT,  1899,  BY  P.  BLAKISTON'S  SON  &  Co. 


COPYRIGHT,  1900,  BY  P.  BLAKISTON'S  SON  &  Co. 


COPYRIGHT,  1904,  BY  P.  BLAKISTON'S  SON  &  Co. 


WM.     F.     FELL     COMPANY 
iUECTROTYPERS.     PHINTCF 


PREFACE  TO  THE  THIRD  EDITION. 


The  second  edition  of  this  work  was  published  in  1900 
and  was  exhausted  two  years  later,  at  which  time  a  few 
changes  were  made  in  the  text  and  the  edition  was  again 
republished  under  date  of  1902.  This  virtually  made  a 
third  edition,  but  it  was  not  so  called.  In  preparing  this, 
the  third,  edition  and  to  enhance  its  value  the  writer  has 
gone  over  the  text  very  carefully  and  added  fifteen  new 
illustrations  together  with  a  description  of  several  new 
instruments  which  have  lately  been  brought  forward  as 
material  aids  in  estimating  refractive  errors. 

120  S.  EIGHTEENTH  ST.,  PHILADELPHIA,  PA. 
August, 


PREFACE 


This  book  has  been  written  at  the  request  of  the  many 
students  who  have  attended  the  author's  lectures  on 
"Refraction"  at  the  Philadelphia  Polyclinic ;  and  while  it 
is  intended  for  all  beginners  in  the  study  of  Ophthalmology, 
yet  it  is  especially  for  those  practitioners  and  students  who 
may  have  a  limited  knowledge  of  mathematics  and  who  can 
not  readily  appreciate  the  classic  treatise  of  Bonders. 

In  the  preparation  of  the  manuscript  and  in  arranging  these 
pages  the  writer  has  planned  to  be  systematic  and  practi- 
cal, so  that  the  student,  starting  with  the  consideration  of 
rays  of  light,  is  gradually  brought  to  a  full  understanding 
of  optics  ;  and  following  this,  he  is  taught  what  is  the  stan- 
dard eye,  and  then  is  given  a  description  of  ametropic  eyes, 
with  a  differential  diagnosis  of  each,  until  finally  he  is  told 
how  to  place  lenses  in  front  of  ametropic  eyes  to  make 
them  equal  to  the  standard  condition. 

By  being  dogmatic  rather  than  ambiguous,  with  occa- 
sional repetitions  to  avoid  frequent  references,  and  by  simple 
explanations  and  a  definite  statement  of  facts,  the  writer  has 
aimed  to  make  the  text  more  concise  and  comprehensive 
than  if  encumbered  with  lengthy  mathematic  formulas  or 
with  any  discussion  of  disputed  points. 

The  chapter  on  Retinoscopy  embraces  descriptions  of  that 
method  of  refracting,  both  with  the  plane  and  with  the 
concave  mirror  ;  but  no  matter  how  carefully  expressed,  the 


Xll  PREFACE. 

student  will  frequently  confuse  the  two,  and  he  is  therefore 
referred  to  the  author's  manual  on  "  Retinoscopy  with  the 
Plane  Mirror." 

Of  the  two  hundred  illustrations  used  to  elucidate  this 
work,  nearly  all  are  newly  made,  and  were  drawn  or  photo- 
graphed by  the  author.  Those  in  colors,  on  page  145,  and 
the  diagrams  of  astigmatic  eyes,  as  also  several  others,  are 
original. 

The  author  desires  to  tender  his  thanks  to  Dr.  Helen 
Murphy,  of  Philadelphia,  and  to  Dr.  J.  Ellis  Jennings,  of 
St.  Louis,  Mo.,  for  many  valuable  suggestions. 

120  S.  i8xH  ST.,  PHILADELPHIA,  PA. 
November,  1899. 


PREFACE  TO  SECOND  EDITION. 


The  first  edition  of  this  work  was  published  in  November, 
1899,  and  it  is  indeed  gratifying  to  the  author  that  the  work 
has  found  such  favor  as  to  call  for  a  second  in  so  short  a 
time. 

In  preparing  this  edition  and  to  make  it  more  lucid  than 
the  first,  the  writer  has  carefully  reviewed  the  original  text 
and  made  some  changes  in  the  phraseology. 

The  writer  takes  this  opportunity  to  thank  his  many 
friends  and  correspondents,  at  home  and  abroad,  for  their 
complimentary  letters  and  reviews,  which  are  gratefully 
appreciated. 

November,  igoo. 


CONTENTS. 


CHAPTER  I. 

PACK 

OPTICS,   ..........................  9 

CHAPTER  II. 

THE  EYE.  —  THE  STANDARD  EYE.  —  THE  CARDINAL  POINTS.  —  VIS- 
UAL ANGLE.  —  MINIMUM  VISUAL  ANGLE.  —  STANDARD  ACUTE- 
NESS  OF  VISION.  —  SIZE  OF  RETINAL  IMAGE.  —  ACCOMMODATION. 

—  MECHANISM  OF  ACCOMMODATION.  —  FAR  AND  NEAR  POINTS. 

—  DETERMINATION  OF  DISTANT  VISION  AND  NEAR   POINT.  — 
AMPLITUDE     OF     ACCOMMODATION.  —  CONVERGENCE.  —  ANGLE 
GAMMA.  —  ANGLE  ALPHA,      ...............  59 

CHAPTER  III. 
OPHTHALMOSCOPE.  —  DIRECT  AND  INDIRECT  METHODS,    .....          gg 

CHAPTER  IV. 
EMMETROPIA.  —  HYPEROPIA.  —  MYOPIA,    ............         I0-, 

CHAPTER  V. 

ASTIGMATISM,  OR  CURVATURE  AMETROPIA.  —  TESTS  FOR  ASTIGMA- 

TISM, .........................  122 

CHAPTER  VI. 
RETINOSCOPY,    .......................         j.g 


CHAPTER  VII. 

MUSCLES, 


CHAPTER  VIII. 

CYCLOPLEGICS.  —  CYCLOPLEGIA.  —  ASTHENOPIA.  —  EXAMINATION  OF 

THE  EYES  .......................         20g 


XIV  CONTENTS. 

CHAPTER  IX. 

PAGE 

How  TO  REFRACT,      228 

CHAPTER  X. 
APPLIED  REFRACTION, 244 

CHAPTER  XI. 
PRESBYOPIA. — APHAKIA. — ANISOMETROPIA. — SPECTACLES,  ....         271 

CHAPTER  XII. 

LENSES,  SPECTACLES,  AND  EYE-GLASS  FRAMES. — How  TO  TAKE 
MEASUREMENTS  FOR  THEM  AND  How  THEY  SHOULD  BE 
FITTED, 297 

INDEX, 3o7 


LIST  OF  ILLUSTRATIONS. 


FIG.  PAGE 

1.  Illustrating  Intensity  of  Light, 10 

2.  Convergent  Pencil,       1 1 

3.  Convergent  Pencil,       II 

4.  Divergent  Pencil, 12 

5.  Reflection, 13 

6.  Reflection  from  Plane  Mirror, 13 

7.  Lateral  Inversion, 14 

8.  Reflection  from  Concave  Mirror, 15 

9.  Erect  Image  Formed  by  Concave  Mirror, 16 

10.  Inverted  Image  Formed  by  Concave  Mirror,       17 

11.  Image  Formed  by  Convex  Mirror, 18 

12.  Perpendicular  to  Plane  Surfaces, 19 

13.  Refraction, 19 

14.  Critical  Angle,      20 

15    and  16.  Angle  of  Refraction, 21 

17.  Density,     .             21 

18.  Index  of  Refraction, 22 

19.  Maximum  Deviation, 23 

20.  Minimum  Deviation, 23 

21.  Angle  of  Deviation, 24 

22.  Displacement, 25 

23.  Centrad, 25 

24.  Prism  Diopter,      25 

25.  Neutralization  of  Prisms, 26 

26.  Correction  of  Diplopia,       29 

27.  28,  and  29.   Convex  Lenses, 30 

30,  31,  and  32.   Concave  Lenses, 31 

33.  Prism  Formation  of  a  Convex  Lens, 31 

34.  Prism  Formation  of  a  Concave  Lens, 31 

35.  Parallel    Rays  Passing  Through  a  Convex  Lens, 32 

36.  Parallel   Rays  Passing  Through  a  Concave  Lens, 33 

37.  Conjugate  Foci, 34 

38.  Ordinary  Foci,      35 

39.  Negative  Focus, 36 

40.  Nodal  Points, 36 

41.  Optic  Center, 37 

42.  Inverted  Image  Formed  by  a  Convex  Lens 38 

43.  •  Erect  Magnified  Image  Formed  by  a  Convex  Lens, 40 

44.  Image  Formed  by  a  Concave  Lens,       40 

45    and  46.   Cylindric  Lenses,           44 

47.  Cylinder  Axis,       44 

48.  Parallel  Rays  Passing  Through  a  Convex  Cylinder, 44 


XVI  LIST   OF    ILLUSTRATIONS. 

FIG.  PACK 

49.  Parallel  Rays  Passing  Through  a  Concave  Cylinder, 45 

50.  Trial  Case,      46 

51  and  52.   Trial-frames, 47  and  48 

53.  Combining  Sphere  and  Cylinder, 50 

54,  55,  and  56.   Finding  Optic  Center  of  a  Lens, 55 

57  and  58.      Finding  Cylinder  Axis, 56 

58,  59,  and  60.   Action  of  a  Cylinder, 56  and  57 

61.  Standard  Eye, bo 

62.  Angle  of  View,      6l 

63  and  64.  Size  of  Retinal  Image, 62 

65.  Minimum  Visual  Angle, 63 

66  and  67.   Five-minute  Angle, 64 

68.  Retinal  Image  in  the  Standard  and  Ametropic  Eyes, 65 

69.  Crystalline  Lens  at  Rest  and  Accommodating,      68 

70.  Accommodation 69 

71.  Hyperopic  Eye  at  Rest, 71 

72.  Myopic  Eye  at  Rest 72 

73.  Randall's  Test-letters 74 

74.  Wallace  Test-letters 75 

75.  Illiterate  Card, 75 

76.  Kindergarten  Card, 76 

77.  Gould's  Test-letters, 77 

78.  Gothic  Type  for  Testing  the  Near  Point, 8l 

79.  Block  Letters  for  Testing  the  Near  Point, 82 

80.  Meter  Angle  of  Convergence, 84 

81.  Angle  Gamma,        85 

82.  Positive  Angle  Gamma, 86 

83.  Negative  Angle  Gamma, 87 

84.  Loring  Ophthalmoscope, 89 

85.  Direct  Ophtha'.moscopy, 90 

86.  Emmetropia  with  the  Ophthalmoscope, 95 

87.  Hyperopia  with  the  Ophthalmoscope, 96 

88.  Myopia  with  the  Ophthalmoscope, 97 

89.  Indirect  Ophthalmoscopy, 99 

90.  Condensing  Lens, 99 

91  and  92.   Luminous  Ophthalmoscope,      102 

93.  Emmetropia,  .        103 

94.  Emmetropic  and  Ametropic  Eyes  23  mm.  Long, 104 

95.  Hyperopic  Eye  at  Rest, 107 

96.  Hyperopic  Eye  Refracted, 107 

97.  Parallel  Rays  Entering  a  Myopic  Eye, 113 

98.  Myopic  Eye  at  Rest,      114 

99.  Myopic  Eye  Refracted,      114 

100.  Astigmatic  Lens, 123 

101.  Simple  Hyperopic  Astigmatism, 126 

102.  Simple  Myopic  Astigmatism 127 

103.  Compound  Hyperopic  Astigmatism, 127 

104.  Compound  Myopic  Astigmatism, 128 

105  and  106.      Mixed  Astigmatism, 129 

107.  Symmetric  Astigmatism 130 

1 08.  Asymmetric  Astigmatism,      130 

109.  Astigmatism  with  the  Rule, 131 


LIST    OF    ILLUSTRATIONS.  XV11 

FIG.  PACK 

no.  Astigmatism  against  the  Rule, 131 

111.  Placido's  Disc, 135 

112.  Stenopeic  Slit, 135 

113.  Green's  Astigmatic  Charts, 137 

114.  Astigmatic  Clock-dial, 138 

115.  Astigmatic  Clock-dial  in  Black, 139 

116.  Author's  Pointed  Line  Test, 141 

117.  Perforated  Disc 142 

118.  Pray's  Letters 142 

119.  Scheiner's  Disc, 143 

1 20.  Scheiner's  Disc  in  Hyperopia,      143 

121.  Scheiner's  Disc  in  Myopia, 144 

122    and  123.   Cobalt-blue  Glass,         145 

124.    Refrangibility  of  Cobalt-blue  Glass, 146 

125    to  136,  inclusive.     The  Diagnosis  of  the  Different  Forms  of  Ame- 

tropia  with  Cobalt-blue  Glass, .' 147 

137.   Thomson's  Ametrometer, 149 

138    and   139.   Ophthalmometer, 150  and  151 

140    and  141.    Mires  or  Targets, 152 

142.  Indirect  Ophthalmoscopy, 155 

143.  Author's  Schematic  Eye, 156 

144.  Point  of  Reversal,       157 

145     and   146.    Author's  Mirror  with  Folding  Handle,      158 

147.  Author's  Iris  Diaphragm  Chimney, 159 

148.  Position  of  Light  and  Mirror,       160 

149.  High  Myopia  as  Seen  with  the  Concave  Mirror, l6l 

150.  Hyperopia  as  Seen  with  the  Concave  Mirror, 161 

151     and  152.    Rate  of  Movement  of  Retinal  Illumination  in  Hyperopia 

and  Myopia, 164  and  165 

153.  Retinal  Illumination  in  Emmetropia, 166 

154.  Band  of    Light, , 169 

155-  Axonometer, 170 

156.  Scissor  Movement, 172 

157.  Positive  Aberration, 173 

158.  Negative  Aberration 173 

159    and   1 60.   Reisner  Retinoscope, 174 

161     and   162.    Luminous  Retinoscope, 175 

163.  Homonymous  Diplopia, 178 

164.  Heteronymous  Diplopia,       179 

165.  Scale  for  Testing  Lateral  Insufficiency, 187 

166    and   167.   Maddox  Rods,      188 

168.  Rotary  Prism  of  Risley, 189 

169.  Phorometer 190 

170.  Strabismometer,      ....  200 

171.  Angle  of  Deviation  in  Strabismus, 201 

172.  Monocular  Blinder, 203 

173.  Worth's  Amblyoscope,      204 

174.  Aphakia,  278 

175     and   176.    Franklin  Bifocals, 284 

177.   Mork's  Bifocals,  284 

178    to   184,   inclusive.     Cement  Bifocals, 285  and  286 

185    and  1 86.  Acromatic  Bifocals, 286 


XV111  LIST    OF    ILLUSTRATIONS. 

FIG.  PACK 

187  and  188.   Solid  Bifocals, 287 

189  to  193,  inclusive.     Half  Lenses 288 

194.  Toric  Lenses, 290 

195  and   196.  Trifocals, 295 

197  to  206,  inclusive.     Different  Sizes  and  Shaped  Lenses,    .      298  and  299 

207.  Measuring  Interpupillary  Distance, 301 

208  and  209    Fitting  of  Spectacle  Bridge, 302 

210.  Measurement  of  Bridge, 303 

211.  Measurement  for  Spectacles, 304 

212.  Measurement  for  Eye-glasses, 304 

213.  Distance  Frames,    .    .             305 

214.  Near  Frames,      . 305 

215.  Measurements  for  Guards, 305 


REFRACTION 


HOW  TO  REFRACT. 


CHAPTER   I. 

OPTICS. 

Optics  (from  the  Greek  OXTO/JLUI,  meaning  "to  see")  is 
that  branch  of  physical  science  which  treats  of  the  nature 
and  properties  of  light.  " 

Catoptrics  (from  the  Greek  xdToxrpoy,  meaning  "  a  mir- 
f  ror")  and    dioptrics   (from    the    Greek  diimrpov,   meaning 
"  to  see  through  ")  are  subdivisions  of  optics  ;  the  former 
treating  of  (incident  and  reflected  rays)  and  the  latter  of  the 
^refraction  of  lightjpassing  through  different  media,  such  as 
air,  water,  glass,  etc.,  but  especially  through  lenses. 

Light. — Light  may  be  defined  as  that  form  of  energy 
which,  acting  upon  the  organs  of  sight,  renders  visible  the 
objects  from  which  it  proceeds.  This  form  of  energy  is 

/  £^  ** 

propagated  in  waves  in  all  directions  from  a  luminous  body, 
.and  with  a  velocity  in  a  vacuum  of  about  186,000  miles  a 

second.     In  the  study  of  a  luminous  body,  such  as  a  candle-, 
lamp-,  or  gas-flame,  the  substance  itself  must  not  be  con- 
sidered as  a  single  source  of  radiation,  but  as  a  collection 
^ 

of  minute  points,  from  every  one  of  which  waves  proceed 

^HMMMfMMpMW* 

in  all  directions  and  cross  one  another  as  they  diverge  from 
their  respective  points.     The  intensity  of  light  deer 


10         REFRACTION  AND  HOW  TO  REFRACT. 

as  the  square  of  the  distance  from  the  light  increases :  for 
example,  if  an  object  is  twice  as  far  from  a  luminous  body 
as  another  of  the  same  size,  it  will  receive  one-fourth  as 
much  light  as  the  latter.  Figure  i  shows  t\vo  cards,  one 
is  twice  as  far  from  the  light  as  the  other  and  receives  only 
one-fourth  as  much  light  as  the  card  nearest  to  the  light. 

Ray. — Ray  (from  "  radius  ")  is  used  in  optics  in  prefer- 
ence  to  wave,  and  /means  the  smallest  subdivision  of  li^ht 

0 

traveling  in  a  straight  line)  Rays  of  light  are  considered 
as  incident,  emergent,  reflected,  refracted,  divergent,  par- 
allel, and  convergent. 


FIG.    I. — Illustrating  Intensity  of  Light. 

Incident  Rays. — Rays  of  light  are  said  to  be  incident 
when  they  strike  the  surface  of  an  object.     (See  Fig.  8.) 
/     Emergent  Rays. — Rays  of  light  are  emergent  when  they 
.'  have  passed  through  a  transparent  substance.     (See  Fig.  1 3 .) 

Reflected  Rays. — Rays  of  light  are  reflected  when  they 
rebound  from  a  polished  surface.  (See  Fig.  8.) 

Refracted  Rays. — A  ray  of  light  undergoes  refraction 
when  it  is  deviated  from  its  course  in  passing  through  any 
transparent  substance. 

Divergent  Rays. — Rays  of  light  proceed   divergently 
from  any  luminous  substance,  but,  in  the  study  of  refrac- 
tion, only  those  which  proceed  from  a  point  closer  than  six 
/meters  are  spoken  of  as  divergent.      (Fig.  I.) 

Parallel  Rays. — The  greater  the  distance  of  any  lumin- 
ous point,  the  more  nearly  do  its  rays  approach  to  paral- 


OPTICS. 


II 


lelism  ;  this  is  evident  in  a  study  of  rays  coming  from  such 
distant  sources  as  the  sun,  moon,  and  stars.  For  all  prac- 
tical purposes  in  the 
study  of  refraction, 
rays  of  light  which 
proceed  from  a  dis- 
tance of  six  meters  or 
more  are  spoken  of 
as  parallel,  although 
this  is  not  an  absolute 
fact,  as  rays  of  light 
at  this  distance  still 
maintain  a  slight 
amount  of  divergence. 
If  the  pupil  of  the 

emmetropic  eye  is  represented  by  a  circular  opening  four 
millimeters  in  diameter,  then  rays  of  light  from  a  luminous 
point  at  six  meters  (6000  mm.)  will  have  a  divergence  of 

when  they  enter  such  a  pupil. 
Convergent  Rays. — Convergent  rays  are  the  result  of  re- 


FIG.  2. — Parallel  Rays  Reflected  by  a  Concave 
Mirror  Forming  a  Convergent  Pencil. 


FIG.  3. — Parallel   Rays   Refracted  by  a  Convex   Lens  Forming  a  Convergent 

Pencil. 

flection  from  a  concave  mirror  or  refraction  through  a  con- 
vex lens.      (See  Figs.  2  and  3.)  y- 
'  A  Beam. — This  is  a  collection  or  series  of  parallel  rays. 
(See  Fig.  3.) 

A  Pencil. — A  pencil  of  light  is/a  collection  of  convergent 


12         REFRACTION  AND  HOW  TO  REFRACT. 

or  divergent  raysj  '  Convergent  rays  are  those  which  tend 
to  a  common  point  (see  Fig.  3),  whereas  divergent  rays  are 
those  which  proceed  from  a  point  and  continually  separate 
as  they  proceed.)  (See  Fig.  4.)  This  point  is  called  the 
radiant  point. 

A  Focus.  —  This  is  the  point  of  a  convergent  or  divergent 
pencil  ;  the  center  of  a  circle  ;  the  point  to  which  converg- 
ing rays  are  directed. 

A  Positive  or  Real  Focus.  —  This  is  the  point  to  -which 

rays  are  directed  after  passing  through  a  convex  lens  or 

after  reflection  from  a  concave  mirror.    (See  Figs.  3  and  2.) 

A  Negative  or  Virtual  Focus.  —  This  is  the  point  from 

which  rays  appear  to 
diverge  after  passing 
through  a  concave  lens  J 
(see  Fig.  36),  or  Rafter 
reflection  from  a  convex 
mirror,  or  after  refraction 
through  a  convex  lens 
when  the  light  or  object 

FIG.  4.—  Illustrating  a  Divergent  Pencil.  js  closer  to  the  lens  than 

its  principal  focus  (see 

Fig.  43),  or  after  j  reflection  from  a  concave  mirror  when 
the  light  or  object  is  closer  to  the  mirror  than  its  principal 
focus,/  (See  Fig.  9.) 

The  principal  phenomena  of  light  are  absorption,  reflec- 
tion, and  refraction. 

Absorption.  —  Rays  of  light  from  the  sun  falling  upon 
the  green  grass  are  partly  absorbed  and  partly  reflected. 
The  grass  absorbs  some  of  the  rays  and  sends  back  or 
reflects  only  those  rays  which  together  produce  the  effect 
of  green.  A  piece  of  red  glass  owes  its  color  to  the  fact 
that  it  transmits  only  that  portion  of  the  light's  rays  whose 
combined  effect  upon  the  retina  is  that  of  red.  (The  relative 


OKJU^/~> 
•  Of  ('.'•;  i/l    V  '•'  I 


OPTICS.  1 3 

proportion  of  absorption  and  reflection  of  rays  of  light 
depends  greatly  upon  the  quality  of  the  suiface — whether 
light  colored  or  polished,  or  dark  colored  or  rough. ^) 

Reflection. — From  the  Latin  rcflectcrc,  "to  rebound." 
This  is  the  sending  back  of  rays  of  light  by  the  surface  on 
which  they  fall  into  the  medium  through  which  they  came. 
\Yhile  most  of  the  rays  falling  upon  the  surface  of  a  trans- 
parent substance  pass  through  it,  with  or  without  change 
'in  their  direction,  yet  some  of  the  rays  are  reflected,  and  it 
is  by  these  reflected  rays  that  surfaces  are  made  visible. 


D 
FK;.  5. 


FIG.  6. 


!A  substance  that  could  transmit  or  absorb  all  the  rays  of 
light  coming  to  it  (if  such  a  substance  existed)  would  be 
invisible.  ( Reflection,  therefore,  always  accompanies  refrac- 
tion, and,  if  one  of  these  disappear,  the  other  will  disappear 
also. ) 

Laws  of  Reflection. — (i)  The  angle  of  reflection  is 
equal  to  the  angle  of  incidence.  (2)  The  reflected  and  in- 
cident rays  are  in  the  same  plane  with  the  perpendicular  to 
the  surface.  (See  Fig.  5.) 

If  A  B  represent  a  polished  surface  and  I  the  incident 
ray,  then  P  D  I  is  the  angle  of  incidence  ;  R  being  the  re- 


REFRACTION  AND  HOW  TO  REFRACT. 


R 

E  F 
LEG 
TION 


1  ~ 

fl 

T    3 

03J 

HOIT 

fleeted  ray,  then  P  D  R,  equal  to  it,  is  the  angle  of  reflection. 
I  D,  P  D,  and  R  D  lie  in  the  same  plane. 

A  reflecting  surface  is  usually  a  polished  surface  (a 
mirror),  and  may  be  plane,  concave,  or  convex. 

Reflection  from  a  Plane  Mirror.— {Rays  of  light  are 
reflected  from  a  plane  mirror  in  the  same  direction  in  which 
they  fall  upon  it :  if  parallel,  convergent,  or  divergent  be- 
fore reflection,  then  they  are  parallel,  convergent,  or  diver- 
gent after  reflection.  An  object  placed  in  front  of  a  plane 
mirror  appears  just  as  far  back  in  the  mirror  as  the  object 

is    in    front  of  it.      (See 
Fig.  6.]} 

A  B  represents  a  plane 
mirror  with  E  F,  rays 
from  the  extremes  of  the 
object  I,  reflected  from 
the  mirror  A  B,  and  meet- 
ing at  the  observer's  eye 
as  if  they  came  from  the 
object  I  in  the  mirror. 
(See  Visual  Angle,  p. 
62.)  The  apparent  dis- 

-7  tance  of  the  object  I  from  the  observer  is  equal  Jo  the 
combined  length  of  the  incident  and  reflected  rays. 

The  appearance  of  an  image  in  a  plane  mirror  is  not 
exactly  the  same  as  that  of  the  object  facing  the  mirror ; 
it  undergoes  what  is  known  as  lateraljm^sion.  This  is 
best  understood  by  holding  a  printed  page  in  front  of  a 
plane  mirror,  when  the  words  or  letters  will  read  from  right 
to  left  (See  Fig.  7.)  An  observer  facing  a  plane  mirror 
and  raising  his  right  hand,  his  image  apparently  raises  the 
left  hand. 

Tilting  a  plane  mirror  gives  an  object  the  appearance  of 


FIG.  7.— Lateral  Inversion. 


moving  in  the  opposite  direction  to  that  in  which  the  mirror 
is  tilted. 

Spheric  Mirrors. — A  spheric  mirror  is  a  portion  of  a 
reflecting  spheric  surface  ;  its  center  of  curvature  is  therefore 
the  center  of  the  sphere  of  which  it  is  a  part.  ('  Spheric  mir- 
rors are  of  two  kinds — concave  and  convex.) 

Reflection  from  a  Concave  Mirror  (Fig.  8). — Parallel 
rays  are  reflected  from  a  concave  mirror,  and  are  brought  to 
a  focus  in  front  of  it.  This  point  is  called  the  principal  focus 
(P.P.).  The^principal  axis  of  a  concave  mirror  is  a  straight 
line  drawn  from  the  mirror  through  the  principal  focus  and 


v; 


/' 


•~— < — 

~^~-  — 

•^.P'F':-^..J     '    ^-  --— — . 


FIG.  8. 


the  center  of  curvatureYi— i),  and(a  secondary  axis  (2',  2', 
2',  2')  is  any  other  straight  line  passing  from  the  mirror  to 
the  center  of  curvature  (C.C.).J  Rays  which  diverge  from 
'any  point  beyond  the  principal  focus  are  reflected  con- 
vergently  (G  J).  Rays  which  diverge  from  any  point  closer 
than  the  principal  focus  are  reflected  divergently  (V  V). 

Images  Formed  by  a  Concave  Mirror.-j^To  find  the 
position  of  an  image  as  formed  by  a  concave  mirror,  two 
rays  may  be  used/  one  drawn  from  a  given  point  on  the 
object  to  the  mirror,  and  parallel  to  its  principal  axis,  and 
reflected  through  the  principal  focus  (P.P.,  Figs.  9  and  10); 
the  other,  the  secondary  axis,  from  the  same  point,  passing 


/S  ^    Ft^ 
/y c*  +* 

1 6         REFRACTION  AND  HOW  TO  REFRACT. 

through  the  center  of  curvature.  The  place  where  the 
secondary  axis  and  the  reflected  ray  or  their  projections  in- 
tersect gives  the  position  of  the  image.  Unlike  the  plane 
mirror,  which  produces  images  at  all  times  and  at  all  dis- 
jtances,  the  concave  mirror  produces  either  an  erect,  virtual, 
'and  enlarged  image,  if  an  object  is  placed  closer  than  its 
principal  focus,  or  an  enlarged  inverted  image  if  the  object 
is  between  the  principal  focus  and  the  center  of  curvature. 

By  withdrawing  the  mirror  in  the  former  instance  the 
erect  image  increases  slightly  in  size,  and  in  the  latter  the 
inverted  image  diminishes  in  size.  /'At  the  principal  focus 
there  is  no  image  formed.) 


FIG.  9. 


Figure  9  shows  an  erect,  virtual,  and  enlarged  image  of 
A  R  which  is  closer  to  the  mirror  than  the  principal  focus. 
Parallel  rays  from  A  and  R  are  reflected  to  the  principal 
focus,  P.P.  Lines  drawn  from  the  center  of  curvature 
through  A  and  R  to  the  mirror  are  secondary  axes  ;  these 
lines  and  those  reflected  to  the  principal  focus  do  not  inter- 
sect in  front  of  the  mirror,  but  if  projected,  will  meet  at  a 
and  r  behind  the  mirror,  forming  a  magnified  image  of 
A  R.  If  the  mirror  is  withdrawn  from  the  object,  the 
erect  magnified  image  will  increase  in  size,  but  at  the  prin- 
cipal focus  no  image  will  be  formed,  as  the  rays  are  reflected 
parallel. 


Figure  10  shows  a  real  inverted  image  of  A  R  at  a  r ; 
A  R  situated  beyond  the  principal  focus.  Lines  drawn 
from  A  and  R  through  C.C.  are  secondary  axes.  Parallel 
rays  from  A  and  R  converge  and  cross  at  the  principal 
focus  (P.F.). 

Where  D  P  and  F  E  intersect  the  secondary  axes,  the  in- 
verted image  a  r  of  A  R  is  situated.  ^Vhen  the  object,  as 
in  this  instance,  is  situated  beyond  the  center  of  curvature, 
the  image  is  smaller  than  the  object. ,  As  the  image  and 
object  are  conjugate  to  each  other,  they  are  interchange- 
able, and  in  such  a  case  the  image  would  be  larger  than 
the  object  and  inverted.  /This  is  always  true  when  the 


FIG.  10. 

object  is  situated  between  the  center  of  curvature  and  the 
principal  focus.  j(vVhen  an  object  is  situated  at  the  center 
of  curvature,  its  image  is  equally  distant  and  of  the  same 
size,  but  inverted. 

Tilting  a  concave  mirror  gives  an  object  placed  inside  of    / 

r  i  • 

its  principal  focus  the  appearance  of  moving  as  the  mirror 
is  tilted  ;  but  if  the  object  is  situated  beyond  the  principal 
focus,  the  object  appears  to  move  in  the  opposite  direction. 
Reflection  from  a  Convex  Mirror. — All  rays  are  re- 
flected divergently  from  a  convex  mirror,  and  parallel  rays 
diverge  as  if  they  came  from  the  principal  focus  situated 
behind  the  mirror  at  a  distance  equal  to  one-half  its  radius 


18 


REFRACTION     AND     HOW     TO     REFRACT. 


of  curvature.      The  principal  focus  of  a   convex  mirror  is 
therefore  negative.      The  foci  of  convex  mirrors  are  virtual. 


are 


Images   Formed   by   a   Convex   Mirror. — These 
^always   virtual,  erect,  and  smaller  than   the  object^)    The 
closer  the  object,  the  larger  the  image  ;  and  the  more  distant 
the  object,  the  smaller  the  image.    (Tilting  a  convex  mirror, 
,the  image  does  not  appear  to  change  position.. 

In  figure  1 1  parallel  rays  from  the  object  A  R  are  reflected 
from  the  mirror  as  if  they  came  from  the  principal  focus  situ- 
ated at  one-half  the  distance  of  the  center  of  curvature,  C.C. 
Lines  drawn  from  the  extremes  of  the  object  to  C.C.  are 


C.C. 


P.F. 


FIG.  ii. 

secondary  axes,  and  f' the  image  is  situated  at  the  point  of 
intersection  of  the  secondary  axes  and  the  rays  from  the 
principal  focus^  and  as  these  meet  behind  the  mirror,  the 
jimage  is  virtual  and  erect. 

Refraction. — From  the  Latin  refrangere,  meaning  "  to 
bend  back  " — /.  e.,  to  deviate  from  a  straight  course.  /Refrac- 
tion may  be  defined  as  the  deviation  which  takes  place  in 
the  direction  of  rays  of  light  as  they  pass  from  one  medium 
into  another  of  different  density)* 

(*  As  ordinarily  understood  in  ophthalmology,  refraction  has  come  to  mean 
the  optic  condition  of  an  eye  in  a  state  of  repose  or  under  the  physiologic  effect 
of  a  cycloplegic. 


OPTICS.  19 

Two  laws  govern  the  refraction  of  rays  of  light : 

1.  A  ray   of  light  passing   from   a   rare  into   a  denser 
medium  is  deviated  or  refracted  toward  the  perpendicular. 

2.  A   ray   of  light    passing    from  a  dense    into  a  rarer 
medium  is  deviated   or  refracted  away  from   the   perpen- 
dicular. 

Aside  from  these  laws,  there  are  other  facts  in  regard 

to  rays  of  light  that  should  have  consideration.      A  ray 

of  light    will    continue    its    straight    course    through   any 

/  number  of  different  transparent  media,  no  matter  what  their 

I  densities,  so  long  as  it  forms  right  angles  with   the  surface 


£   ICE 

\ 

/  FLINT  GLASS 

>   CROWN    " 

3    PLATE       " 

$ 

> 
FIG.  12 

or  surfaces.  (Such  a  ray  is  spoken  of  as  the  normal  or  per- 
pendicular} {such  surfaces  are  plane,  the  surfaces  and  per- 
pendicular forming  right  angles.)  (See  Fig.  12.)  In  any 

lease  of  refraction   the  incident  and   refracted   rays  may  be 

[supposed  to  change  places. 

Figure  1 3  shows  the  perpendicular  (P  P)  to  a  piece  of 
plate  glass  with  plane  surfaces.  The  ray  in  air  incident  at 
O  on  the  surface  S  F  is  bent  in  the  glass  toward  the  per- 
pendicular, P  P.  The  dotted  line  shows  the  direction  the 
ray  would  have  taken  had  it  not  been  refracted.  As  the 
ray  in  the  glass  comes  to  the  second  surface  at  R,  and 


2O         REFRACTION  AND  HOW  TO  REFRACT. 

passes  into  a  rarer  medium,  it  is  deviated  from  the  perpen- 
dicular, P  P.  The  ray  now  continues  its  original  direction, 
but  has  been  deviated  from  its  course  ;(it  has  undergone 
lateral  displacement) 

Critical  Angle  or  Limiting  Angle  of  Refraction. — This 
is /the  angle  of  incidence  which  just  permits  a  ray  of  light 
in  a  dense  medium  to  pass  out  into  a  rare  medium)  The 

^  J 

(size  of  the  critical  angle  depends  upon  the  index  of  refrac- 
tion of  different  substances. \  Figure  14  shows  an  electric 
light  suspended  in  water.  The  ray  from  this  light  which 
forms  an  angle  of  48°  35'  with  the  surface  of  the  water 


FIG.  14. — Critical  Angle. 

will  be  refracted  and  pass  out  of  the  water,  grazing  its  sur- 
face ;  but  those  rays  which  form  an  angle  greater  than 
48°  35'  will  not  pass  out  of  the  water,  but  will  be  reflected 
back  into  it.  /The  surface  separating  the  two  media  be- 
comes a  reflecting  surface  and  acts  as  a  plane  mirror.^ 

The  critical  angle  for  crown  glass  is  40°  49'. 

Index  of  Refraction. — By  this  is  (meant  the  relative 
density  of  a  substance  or  the  comparative  length  of  time 
required  for  light  to  travel  a  definite  distance  in  different 
substances.  The  ('absolute  index  <>f  n-fraction  is  the 
density  or  refractive  power  of  any  substance  as  compared 


OPTICS. 


21 


with  a  vacuum.)  According  to  the  first  law  of  refraction,  a 
ray  of  light  passing  from  a  rare  into  a  dense  medium  is 
refracted  toward  the  perpendicular ; (in  other  words,  the 
angle  of  refraction  is  smaller,  under  these  circumstances, 
than  the  angle  of  incidence. J)  In  the  study  of  the  compara- 
tive density  of  any  substance  it 

fwill  be  seen  that  the  angle  of 
refraction  is  usually  smaller  the 
more  dense  the  substance^  this 
is  well  illustrated  in  figures  15 
and  1 6. 
I  The  greater  the  density,  the 

/slower  the  velocity  or  the  more 

effort  apparently  for  the  wave  or  ray  to  pass  through 
the  substance.  This  is  illustrated  in  figure  17,  where  a 
ray  or  wave  of  light  is  seen  passing  at  right  angles  through 
different  media.  A  ray  passes  through  a  vacuum  without 
apparent  resistance,  but  in  its  course  through  air  it  is 
slightly  impeded,  so  that/air  has  an  index  of  refraction  of 


FIG.  15. 


FIG.  16. 


/     /     /     /     / 

/ 

Ice 

'X_/'X_/-\_x 

Glass 

^\^^^ 

Diamond 

Vacuum 

Air 

FIG.  17. 

1.00029+  when  compared  with  a  vacuum);  but  as  this  is  so 
slight,  air  and  a  vacuum  are  considered  as  one  for  all  pur- 
poses in  refraction.      To  find  the  index  of  refraction  of  any 
substance  as  compared  witrravacuun^orjur^tis  neces-  I 
sary  to  divide  the  sine  of  the  angle  of  incidence  by  the  sine  I 
of  the  angle  of  refraction. 


22 


REFRACTION    AND    HOW    TO    REFRACT. 


In  figure  18  the  angle  of  incidence  P  C  I  is  the  angle 
formed  by  the  incident  ray  I  with  the  perpendicular,  P  P. 
The  angle  of  refraction  P  C  R  is  the  angle  formed  by  the 
refracted  ray  with  the  perpendicular,  P  P.  Drawing  the 
circle  P  H  P  O  around  the  point  of  incidence  C,  and  therx  «p* 

drawing  the  sines^TD 
X.  and  B  F,  perpen- 
diculars  to  the  per- 
pendicular P  P.raivide 
the  sine  D  X  of  the 
angle  of  incidence  by 
the  sine  F  B  of  the 
angle  of  refraction  to 
obtain  the  index  of 
refraction)  in  this  in- 
stance, water  as  com- 
pared with  air.  D  X 
equaling  4  and  F  B 
equaling  3,  then  4  di- 
vided by  3  will  equal  £,  or 

1.33  -J-,  the  index  of  refraction  of  water  as  compared  with  air. 
(To  find  the  index  of  refraction  of  a  rare  as  compared 
with  a  dense  substance,  divide  the  sine  of  the  angle  of 
refraction  by  the  sine  of  the  angle  of  incidence  —  /.  c.,  air 
as  compared  with  water  would  be  ^,  or  0.75.  \ 

INDEXES  OF  REFRACTION. 


Water             .            •.    .    .    . 

.177 

Cornea            .        .    .            .        

.7777 

,  Crown  plass. 

.C 

Flint  elass 

.58 

Crystalline  lens,  nucleus,     

.4.7 

"           "      intermediate  layer,  

.41 

"           "      cortical  layer.    , 

•  3Q 

OPTICS.  23 

A  prism  is  a  wedge-shaped  portion  of  a  refracting 
medium  contained  between  two  plane  surfaces.  The  sides 
of  a  prism  are  the  inclined  surfaces.  The  apex  is  where 
the  two  plane  surfaces  meet.  The  base  of  the  prism  is  the 
thickest  part  of  the  prism.  The  refracting  angle  is  the 
angle  at  which  the  sides  come  together. 

Position  of  a  Prism. — When  a  prism  is  placed  in  front 
of  an  eye,  its  position  is  indicated  or  described  by  the  direc- 
tion in  which  its  base  is  situated  :  base  down  means  that  the 
thick  part  of  the  prism  is  toward  the  cheek  ;  base  up  means 
that  the  thick  part  of  the  prism  is  toward  the  brow  ;  base 
in  means  that  the  thick  part  of  the  prism  is  toward  the 


IMG.  19.  FIG.  20. 

nose  ;  and  base  out  means  that  the  thick  part  of  the  prism 
is  toward  the  temple. 

Prismatic  Action. — Rays   of  light  passing  through  a 

f  \ 

prism  are^a.hvays'i-efracted  toward  the  basejbf  the  prism. 
If  an  incident  ray  is  perpendicular  to  the  surface  of  a  prism, 
there  will  be  only  one  refraction)  and  that  takes  place  at 
the  point  of  emergence.  ^The  angle  of  incidence  in  this 
instance  will  equal  the  angle  of  the  prism,  and  the  maximum 
deviation  takes  place,  as  all  the  refraction  is  done  at  one 


surface./ 

In   figure  19  the  incident  ray  (I)  is  perpendicular  to  the 
surface  A  B,  and  is  not  refracted  until  it  comes  to  the  sur- 


24         REFRACTION  AND  HOW  TO  REFRACT. 

face  A  C  at  E,  when  it  is  bent  toward  the  base  B  C,  all  the 
refraction  taking  place  at  the  surface  A  C. 
\      If  an  incident  ray  forms  an   angle  other  than   a  right 
1  ingle  with  the  first  surface  of  the  prism,  then  it  will  be  re- 
iiracted  twice — as  it  enters  and  as  it  leaves  the  prism. 
^In  figure  20  X  N  is  the  perpendicular  to  the  surface  A  B 
The  ray  (I)  incident  at  N  is  refracted  toward  this  perpen- 
dicular and  follows  the  course  N  E  inside  of  the  prism. 
On  emergence  it  is  refracted  from  the  perpendicular  E  P  of 
J;he  surface  A  C,  and  in  the  direction  of  the  base  of  the  prism.  ) 
/  If  the  incident  ray  (I)  so  falls  upon  the  surface  A  B  that 
the  refracted  ray  (N  E)  is  parallel  to  the  base  (B  C),  and 
the   emergent   ray  is   such   that   the   angle   of  emergence 
equals  the  angle  of  incidence  (I  N  X),  as  in  this  instance,  then 
the  angles  of  incidence  and  of  emer- 
gence are  equal,  and  the  deviation   is 
at  a  minimum,  or  the  least  possible.  J 
Angle  of  Deviation  (Fig.    21). — 
This  is  the  angle  formed  between  the 
directions  of  the  incident  and  emergent 


rays,  and   measures   the   total   devia- 
FlG  2I  tion.     In   all    prisms   of  ten   degrees 

or  less  the  angle  of  deviation  is  equal 


,7\ 


to  half  the  angle  of  the  prism,  but  in  prisms  of  more  than 
ten  degrees  the  angle  of  deviation  increases. 

Summary. — Prisms  do   not  cause  rays  of  light  to  con-      ^ 
verge  or  to  diverge  ;  rays  that  are  parallel  before  refraction 
are  parallel  after  refraction.    (Therefore,  prisms  do  not  form 
images  ;  prisms  have  no  foci.y 

Effect  of  a  Prism. — An  object  viewed  through  a  prism 
has  the  appearance  of  being  displaced,  and  in  a  direction 
opposite  to  the  base — /.  e.,  toward  the  apex. 

Rays  from  the  object  (X,  Fig.  22)  strike  the  prism  at  C, 


OPTICS. 


undergo  double   refraction,  and,  falling  upon  the  retina  of 
the  eye,  are  projected  back  in  the  direction  in  which  they 
were  received,  and  the  apparent  po- 
sition »of  X  is  changed  to  X',  away 
from   the   base    of   the    prism    and 
toward  the  apex. 

Numbering  of  Prisms. — Form- 

ytrly,  prisms  were  numbered  by  their 
refracting  angles ;  now,  however, 
two  other  methods  are  in  use : 
jDennet's  method,  known  as  the  centrad 

'/method,  known  as  the  prism- diopter. 

Dennett's  Method  (Fig.  23). — The  unit,  or  centrad  (ab- 
breviated V  ),  is  a  prism  that  will  deviate  a  -ray  of  light  the 
Y^-jj-  part  of  the  arc  of  the  radian.  This  is  calculated  as 
follows  :  As  much  of  the  circumference  of  a  circle  is  taken 
as  will  equal  the  length  of  its  radius  of  curvature  ;  this  is 
^called  the  arc  of  the  radian,  and  equals  57.295  degrees.; 
The  arc  of  the  radian  is  then  divided  into  100  parts.  /A 


FIG.  22. 


and   Prentice's 


Radius 
FIG.  23. 


I  Meter 
FIG.  24. 


prism,  base  clown,  at  the  center  of  curvature  that  will  devi-  /[> 
ate  a  ray  of  light  downward  just  T^-$  part  of  the  arc  of  the 
radian  is  a  one  centrad,  and  equals  T^g-  of  57.295  degrees, 
or  0.57295  of  a  degree.    ) 


26 


REFRACTION     AND     HOW     TO     REFRACT. 


9876543210 


Ten  centrads  will  deviate  a  ray  of  light  ten  times  as 
much  as  one  centrad,  or  10  X  0.57295  =  5.7295  degrees, 
etc. 

Prentice's  Method  (Fig.  24). — The  unit,  or  prism-diopter 
(abbreviated  P.D.,or/\),is  a  prism 
that  will  deviate  a  ray  of  light 
>  just  i  cm.  for  each  meter  of  dis- 
tance— that  is,  the  -^  part  of 
the  radius  measured  on  the  tan- 
gent. The  deviation  always  be- 
ing i  cm.  for  each  meter  of  dis- 
tance, i  P.  D.  will  deviate  a  ray  of 
light  2  cm.  for  2  meters  of  dis- 
tance ;  3  cm.  for  3  meters,  etc. 
The  comparative  values  of  cen- 
trads and  prism-diopters  is  quite 
uniform  up  to  20,  but  above  20 
the  centrad  is  the  stronger. 

Neutralization  of  Prisms. — 
Knowing  that  rays  of  light  are  de- 
viated by  centrads  and  prism- 
diopters  up  to  20,  in  the  ratio  of 
i  cm.  for  each  meter  of  distance, 
then  tc^  find  the  numeric  strength 
of  any  prism  all  that  is  necessary 
is  to  hold  the  prism  over  a  series 
of  numbered  parallel  lines,  sepa- 
rated by  an  interval  of  i  cm.  or 
fraction  thereof,  and  note  the 

amount  of  displacement)  For  example,  figure  25  shows  a 
series  of  vertical  lines  */j  of  a  cm.  apart,  and  numbered  from 
o  to  9;  an  X  is  placed  at  the  foot  of  the  o  line.  Holding  a 
prism,  base  to  the  right,  at  a  distance  of  ^  of  a  meter  (as 


98765 


I   3 

2 

: 

: 
D 

X 

FIG.  25. 


OPTICS. 


the  lines  are  ^  of  a  cm.  apart)  and  looking  through  the  prism 
at  the  X  on  the  o  line,  it  will  be  seen  that  the  X  has  been 
displaced  to  the  line  to  the  left  corresponding  to  the  number 
of  centrads  or  prism-diopters  in  the  prism  ;  in  this  instance 
three. 

TABLE  SHOWING  THE  EQUIVALENCE  OF  CENTRADS  IN  PRISM-DIOPTERS 

AND  IN  DEGREES  OF  THE  REFRACTING  ANGLE  (INDEX  OF 

REFRACTION  1.54). 


'TO 


CENTRADS. 

PRISM-DIOPTERS. 

REFRACTING  ANGLE. 

I. 

I. 

I°.oo 

2. 

2.0001 

2°.  12 

3- 

3.0013 

3°.i8 

4- 

4.OO28 

4°-23 

5- 

5-0045 

5°.  28 

6. 

6.0063 

6°.32 

7- 

7.0II5 

7°-35 

8. 

8.0172 

8°.38 

9- 

9.0244 

9°-39 

10. 

10.033 

10°.  39 

ii. 

II.O44 

ii°.37 

12. 

12.057 

i2°-34 

13- 

I3-074 

I3°.29 

14. 

14.092 

14°.  23 

15- 

15.114 

I5°.i6 

16. 

16.138 

i6°.o8 

17- 

17.164 

i6°.98 

1  8. 

18.196 

i7°.85 

19. 

19.230 

i8°.68 

20. 

20.270 

i90-45 

25- 

25-55 

23°-43 

30- 

30.934 

26°.8i 

35- 

36.50 

29°.  72 

40. 

42.28 

32°.i8 

45- 

48.30 

34°.  20 

50. 

54-514 

35°-94 

60. 

68.43 

38°-3i 

70. 

84.22 

39°-73 

80. 

102.96 

40°.  29 

90. 

I26.OI 

40°-49 

100. 

155-75 

39°.  H 

>rism  may  be  ncutralix.cd  by  placing  another  prism 
in  apposition  to  it,  with  their  bases  oppositejso  that  in  look- 


/V""^  •»-> 
L-^-  4yt**4&**. 

V*y  /4<t-w-»~^~ 
28    __     REFRACTION  AND  HOW  TO  REFRACT. 

ing  through  the  two  prisms  at  a  straight  line,  no  matter  at 
what  distance,  the  straight  line  will  continue  to  make  one 
straight  line  through  the  prisms  ;  the  strength  of  the  neu- 
tralizing prism  will  equal  the  strength  of  the  prism  being 
neutralized. 

Uses  of  Prisms. — I.  To  detect  malingerers  who  profess 
.-•"/  *"' 

monocular  blindness  so  as  to  obtain  damages  for  supposed 

injuries,  or  who  wish  to  escape  war  service,  or  those  cases 
of  hysteric  blindness  wishing  to  create  sympathy.  This 
"~test  or  use  of  a  prism  is(_known  as  the  diplopia  test^)  and  is 
practised  as  follows  :  A  seven  P.  D.,  base  up  or  down,  with 
a  blank  are  placed  in  the  trial-frame  corresponding  to  the 
"  blind  "  eye  ;  nothing  is  placed  in  front  of  the  seeing-  eye.; 
the  trial -frame,  thus  armed  (without  fhe  patient  seeing  what 
is  being  done),  is  placed  on  the  patient's  face  and  he  is  in- 
structed to  read  the  card  of  test-letters  on  the  wall  across 
the  room.  While  he  is  thus  busy  reading,  and  purposely 
contradicted  by  the  surgeon,  so  as  to  get  his  mind  from  his 
condition,  the  surgeon  suddenly  removes  the  blank  from 
the  "  blind  "  eye.  The  patient  exclaiming  that  he  sees  two 
cards  and  two  of  all  the  letters  proves  the  deception. 

2.  ^Occasionally,  to  counteract  the  effects  of  strabismus, 
or  diplopia  due  to  a  paralysis  of  one  or  more  of  the  extra- 
ocular  musclesj  For  example  :  A  patient  looking  at  a 
point  of  light  focused  on  the  macula  (M)  of  the  left  eye 
(L),  the  right  eye  being  turned  in  toward  the  nose,  receives 
the  rays  upon  the  retina  to  the  nasal  side  of  the  macula, 
and  hence  projects  the  rays  outward  to  the  right,  giving  a 
false  image  to  the  right  side  ;  a  prism  of  sufficient  strength 
is  then  placed  with  its  base  toward  the  temple  (base  out) 
over  the  right  eye,  so  that  the  rays  from  the  light  may  fall 
upon  the  macula  (M),  and  the  diplopia  will  be  corrected 
(See  Fig.  26.) 


OPTICS.  29 

3.  To  test  the  strength  of  the  extra-ocular  muscles  :  A 
patient  looking  with  both  eyes  at  a  distant  point  of  light 
is  made  to  see  one  light  just  above  another  by  placing  a 
3  P.  D.,  base  down  or  up,  before  either  eye,  and  if  a  2^ 
P.  D.  did  not  produce  diplopia  when  similarly  placed,  the 
strength  of  his  vertical  recti  is  then  represented  by  2^ 
P.  D.  The  strength  of  the  prism  placed  base  in  which, 


FIG.  26. 

if  increased,  would  produce  diplopia  is  the  strength  of  the 
externi ;  and  the  strength  of  the  prism  or  prisms  placed 
base  outward  which,  if  increased,  would  produce  diplopia 
is  the  strength  of  the  interni. 

4.   For  exercise  of  weak  muscles.     (See  p.  191.) 
Lenses. — A  lens  is  a  portion  of  transparent  substance 
(usually   of    glass)   having   one    or  both   surfaces   curved. 
There  arc  t\v<>  kinds  of  lenses — spheric  and  cylindric. 


3O         REFRACTION  AND  HOW  TO  REFRACT. 

Spheric    Lenses. — Abbreviated    S.    or    sph.       Spheric 
lenses  are  so  named  because  their  curved  surfaces  are  sec- 
tions  of   spheres.      A   spheric  lens  is  one   which    refracts 
•-rays  of  light  equally  in    all  meridians   or  planes.      Spheric 
lenses  are  of  two  kinds — convex  and  concave. 

A  convex  spheric  lens  is  thick  at  the  center  and  thin 
at  the  edge.  (Figs.  27,  28,  29.)  The  following  are  synony- 
mt>us  terms  for  a  convex  lens  :  (i)  Plus  ;  (2)  positive  ;  (3) 
collective  ;  (4)  magnifying.  A  convex  lens  is  denoted  by 
the  sign  of  plus  (  +  ). 

Varieties  or  Kinds  of  Convex  Lenses. — 
I.  Planoconvex,  meaning  one  surface  flat  and  the  other 
convex.     It   is   a   section    of  a 
\  A  V         sphere.     (See  Fig.  27.) 

\  /  \  \\  2.  Biconvex,  also  called  con- 

vexoconvex  or  bispheric,  for  the 
reason  that  it  is  equal  to  two 
planoconvex  lenses  with  their 

/  \l  I/         plane    surfaces    together.     (Fig. 

FIG.  27.      FIG.  28.      FIG.  29.       28.) 

3.    Concavoconvex.     This  lens 

has  one  surface  concave  and  the  other  convex,  the  convex 

surface  having  the  shortest  radius  of  curvature.     (Fig.  29.) 

^  The  following  are  synonymous  terms  for  a  concavoconvex 

lens  :  (i)  Periscopic  ;  (2)  convex  meniscus  ;  (3)  converging 

meniscus  (meniscus  meaning  a  small  moon).     (See  Fig.  29.) 

.  /  A  periscopic  lens  enlarges  the  field  of  vision,  and  is  of 

/  especial  service  in  presbyopia. 

A  Concave  Spheric  Lens. — Such  a  lens  is  thick  at 
the  edge  and  thin  at  the  center.  (Figs.  30,  31,  32.)  The 
following  are  synonymous  terms  for  a  concave  lens:  (i) 
Minus ;  (2)  negative ;  (3)  dispersive ;  (4)  minifying.  A 
concave  lens  is  denoted  by  the  sign  of  minus  ( — ). 

.     ^^   ' 


OI'TICS. 


Varieties  or  Kinds  of  Concave  Lenses. — 

1.  J^anoconcai'C,  meaning  one  surface  flat  and  the  other 
concave.      (Fig.  30.) 

2.  Biconcave,  also  called   concavoconcave  or  biconcave 
spheric,  for  the  reason   that  it  is 

equal  to  t\vo  planoconcave  lenses 
with  their  plane  surfaces  to- 
gether. (Fig.  31.) 

3.  Conyexoconcavf.     This  lens 
has  one  surface  convex  and  the 
other  concave,  the  concave  sur- 
face  having  the  shortest  radius 
of  curvature.      (Fig.    32.)     The 

following  are  synonymous  terms  for  a  concavoconvex  lens  : 
(i)  Concave  meniscus  ;  (2)  diverging  meniscus  ;  (3)  peri- 
scopic. 


17  V7 


A 


FIG.  30.      FIG.  31.       FIG.  32. 


FIG.  33. 


FIG.  34. 


(A.  spheric  lens  may  be  considered  as  made  up  of  a  series 
of  prisms  which  gradually  increase  in  strength  from  the 
center  to  the  periphery,  no  matter  whether  the  lens  be  con- 
cave or  convex.  / 


32         REFRACTION  AND  HOW  TO  REFRACT. 

In  the  convex  sphere  the  bases  of  the  prisms  are  toward 
the  center  of  the  lens,  whereas  in  the  concave  the  bases  of 
the  prisms  are  toward  the  edge.  (See  Figs.  33,  34.) 

Knowing  that  a  prism  refracts  rays  of  light  toward  its 
base,  it  may  be  stated  as  a  rule  that  every  lens  bends  rays 
of  light  more  sharply  as  the  periphery  is  approached — i.  c., 
at  the  periphery  the  strongest  prismatic  effect  takes  place. 

Lens  Action. -£-As  a  ray  of  light  will  travel  in  a  straight 
line  so  long  as  it  continues  to  form  right  angles  with  sur- 
faces) then  the  ray  A  in  figure  35  passes  through  the  bicon- 
vex lens  unrefracted,  or  without  any  deviation  from  its 
course  whatsoever,  for  at  its  points  of  entrance  and  emer- 


FIG.  35. 


gence  the  surfaces  of  the  lens  are  plane  to  each  other.  This 
ray  is  called  the  axial  ray,  and  /the  line  joining  the  centers 
of  curvature  of  the  two  surfaces  i 


The  axis  of  a  planoconvex  or  planoconcave  lens  is  the  liiu 
drawn  through  the  center  of  curvature  perpendicular  to  the 
plane  surface. 

The  ray  B  in  figure  35,  though  parallel  to  the  ray  A, 
forms  a  small  angle  of  incidence,  and  must,  therefore,  be 
refracted  toward  the  perpendicular  to  the  surfaces  of  the 
lens,  and,  passing  through  the  lens,  will  meet  the  axial  ray 
at  P.P.  The  rays  C,  D,  and  K,  also  parallel  to  A  and  B, 
form  progressively  larger  angles  with  the  surface  of  the  lens, 


•• 


OPTICS. 


33 


and  finally  meet  the  axial  ray  at  P.P.  <  It  will  be  seen  at 
once  that  the  rays  all  meet  at  P.P.,  showing  the  progres- 
sively stronger  prismatic  action  that  takes  place  as  the  per- 
iphery of  the  lens  is  approached..' 

In  figure  36  we  have  similar  rays,  A,  B,  C,  D,  and  E, 
passing  through  a  con- 
cave lens.  The  axial 
ray  A  passes  through 
the  centers  of  curvature 
unrefracted,  but  the  rays 

B,  C,  D,  and  E  are  pro- 
gressively refracted  more 

and  more  as  the  periph-  FIG.  36. 

ery  is  approached.     The 

ray  E  in  each  instance  is  refracted  the  most. 

The  action  of  a  convex  lens  is  similar  to  that  of  a  concave 
mirror,  and  the  action  of  a  concave  lens  is  similar  to  that  of 
a  convex  mirror. 

Principal  Focus. — The  principal  focus  of  a  lens  may  be 
defined  (i)  as  the  point  where  parallel  rays,  after  refrac- 
tion, come  together  on  the  axial  ray  ;  or  (2)  as  the  shortest 
focus  ;  or  (3)  as  the  focal  point  for  parallel  rays. 

Focal  Length. — This  is  (the  distance  measured  from  the 
optic  center  to  the  principal  focus.  £The  principal  focus 
of  an  equally  biconvex  or  biconcave  lens  of  crown  glass  is 
situated  at  about  the  center  of  curvature  for  either  surface 
of  the  lens.)^A.  lens  has  two  principal  foci,  an  anterior  and 
a  posterior,  according  to  the  direction  from  which  the  par- 
allel rays  come,  or  as  to  which  radius  of  curvature  is  re- 
ferred to.)  (Seep.  60.)  Figure  35  shows  parallel  rays,  B, 

C,  1),  and  E,  passing  through  a  convex    lens  and   coming 
to  a  focus  on  the  axial  ray  (A)  at  P.  F.  ;  and  as  the  path  of 
a  ray  passing  from   one  point  to  another  is  the   same,  no 


34         REFRACTION  AND  HOW  TO  REFRACT. 

matter  what  its  direction,  then  if  a  point  of  light  be  placed 
at  the  principal  focus  of  a  lens,  its  rays  will  be  parallel  after 
passing  back  through  the  lens.  This  is  equivalent  to  what 
'  takes  place  in  the  standard  or  emmetropic  eye.  An  eye,  in 
other  words,  which  has  its  fovea  situated  just  at  the  princi- 
pal focus  of  its  dioptric  media,  such  an  eye  in  a  state  of  rest 
receives  parallel  rays  exactly  at  a  focus  upon  its  fovea,  and 
therefore  is  in  a  condition  to  project  parallel  rays  outward. 
Conjugate  Foci. —  Conjugate  meaning  "yoked  to- 
gether." ^The  point  from  which  rays  of  light  diverge 
(called  the  radiant)  and  the  point  to  which  they  converge 
(called  the  focus)  are  conjugate  foci  or  points.  )  For  in- 
stance, in  figure  37  the  rays  diverging  from  A  and  passing 


FIG.  37. 

through  the  lens  converge  to  the  point  B  ;  then  the  points 
A  and  B  are  conjugate  foci.  They  are  interchangeable,  for 
if  rays  diverged  from  B,  they  would  follow  the  same  path 
back  again  and  meet  at  A.  The  path  of  the  ray  C  C'  is 
the  same  whether  it  passes  from  A  to  B  or  from  B  to  A  : 
there  is  no  difference.  It  is  by  the  affinity  of  these  points 
for  each  other,  with  respect  to  their  positions,  that  they  are 
called  conjugate. 

The  conjugate   foci  are   equal  when   the  point  of  diver- 

Igence  is  at  twice  the  distance  of  the  principal  focus.     The 

•v  equivalent  to  conjugate  foci  is  found  in  the  long  or  myopic 

eye  ;  an  eye,  in  other  words,  which  has  its  fovea  situated 

further  back  than  the  principal  focus  of  its  dioptric  media, 

the  result  being  that  rays  of  light  from  the  fovea  of  such  an 


OPTICS.  35 

eye  would  be  projected  convergently  after  passing  out  of  the 

eye,  and  would  meet  at  some  point  inside  of  infinity.      In 

other  words,  only  those  rays  which  have  diverged  from  some 

/  point  insidq  of  six  meters  will  focus  upon  the  fovea  of  this 

/  long  eye.  (  The  fovea  of  the  myopic  eye  represents  a  con- 

I  jugate  focus./  A  myopic  eye  is  in  a   condition  to  receive 


divergent  rays  of  light  at  a  focus  on  its  retina  and  to  emit 
convergent  rays. 

Ordinary  Foci. — When  rays  of  light  diverge  from  some 
point  inside  of  infinity  (six  meters)  they  will  be  brought  to 
a  focus  at  some  point  on  the  other  side  of  a  convex  lens, 
beyond  its  principal  focus  ;  this  point  is  called  an  ordinary 
focus.  (A  lens  may  have  many  foci,  but  only  two  principal 


FIG.  38. 

fociJ  The  further  away  from  a  lens  the  divergent  rays  pro- 
ceed, the  nearer  to  the  principal  focus  on  the  other  side  of 
the  lens  will  they  converge.  As  the  divergent  rays  are 
brought  closer  to  the  lens  they  reach  a  point  where  they 
will  not  focus,  but  will  pass  parallel  after  refraction.  c_This 
point  is  the  principal  focusj^  (See  Fig.  38.)  (^A  lens,  there- 
fore, has  as  many  foci  as  there  are  imaginary  points  on  the 
axial  ray  between  the  principal  focus  and  infinity/) 

(When  rays  of  light  diverge  from  some  point  closer  to  a 
lens  than  its  principal  focus,  they  do  not  converge,  but, 
after  refraction,  continue  divergently  ;  their  focus  now  is 
negative  or  virtual,  and  is  found  by  projecting  these  diver- 
gent rays  back  upon  themselves)  to  a  point  on  the  same 


30         REFRACTION  AND  HOW  TO  REFRACT. 

side  of  the  lens  from  which  they  appeared  to  come.      (See 

Fig-  39-) 

This  is  the  equivalent  of  what  takes  place  in  a  short  or 

hyperopic  eye,  an  eye  which  has  its  macula  closer  to  its 
dioptric  media  than  its  principal  focus.     In  a  state  of  rest 


FIG.  39. 

the  fovea  of  such  an  eye  would  project  outward  divergent 
rays,  and  would  be  in  a  position  to  receive  only  convergent 
rays  of  light  at  a  focus  upon  its  fovea. 

Secondary  Axes. — In  the  study  of  the  direction  of  a  ray 
of  light  passing  through  a  dense  medium  with  plane  sur- 


FIG.  40. 


faces,  it  was  found  that  it  underwent  lateral  displacement 
(see  Fig.  1 3),  and(so  in  lenses  there  is  a  place  where  rays 
undergo  lateral  displacement)  Figure  40  shows  a  convex 
lens  of  considerable  thickness,  and  on  each  side  is  drawn  a 
radius  of  curvature  (C  C).  The  ray  indicated  by  the  arrow 


7\ 


OPTICS.  37 

passed  through  the  two  surfaces,  has  undergone  lateral 
displacement,  but  continues  in  its  original  direction  ;  such 
rays  are  called  secondary  rays  or  axes.  The  incident  ray 
is  projected  toward  N1  in  the  lens  on  the  axial  ray,  and 
the  emergent  ray,  if  projected  backward,  would  meet  the 
axial  ray  at  N2.  (These  points  on  the  axial  ray  are  such 
that  a  ray  directed  to  one  before  refraction,  is  directed  to 
the  other  after  refraction.  The  points  N1  and  N2  are  ^ 
spoken  of  as  nodafpoints.  /Every  lens,  therefore,  has  two 
nodal  points,  but  in  thin  lenses  the  deviation  of  the  second- 
ary rays  is  so  slight  that,  for  all  practical  purposes,  only 


FIG.  41.  ~>^tt^^    Jr>*-W^    /  c 

^    "^ 

^one  nodal  point  is  recognized.  \  It  is  spoken  of  as  the  optic       /V^. 

'  center.  When  writing  prescriptions  for  glasses,  this  point  of 
having  the  lenses  or  glasses -made  as  thin  as  possible,  must 
be  borne  in  mind. 

/  Optic  Center. — This  term  is  used  synonymously  with 
nodal  point,  and/ is  the  point  where  the  secondary  rays  (sia.\\\ 
Fig.  4 1 )  cross  the  axial  ray.  It  is^not  always  the  geometric 
center)  /Rays  of  light  crossing  the  optic  center  in  thin  lenses 
are  not  considered  as  undergoing  refraction^)  (See  Fig.  41.) 
Action  of  Concave  Lenses. — Rays  of  light  passing 
through  a  concave  lens,  no  matter  from  what  distance,  are 

^always  refracted  divergently,  and  its  focus  is,  therefore, 
always  negative" or  virtuah)and  is  found  by  projecting  these 


35         REFRACTION  AND  HOW  TO  REFRACT. 

divergent  rays  backward  in  the  direction  from  which  they 

appear  to  come  until  they  meet  at  a  point  on  the  axial  ray. 

I  The  principal  focus  and  conjugate  foci  of  concave  lenses 

I  are  found  in  the  same  way  as  in  convex  lenses.     (See  Figs. 

'  36,  44-) 

Images  Formed  by  Lenses.— ^An  image  formed  by  a 

lens  is  composed  of  foci,  each  one  of  which  corresponds 
to  a  point  in  the  object.  Images  are  of  two  kinds — real  and 
virtual. 

A  Real  Image. — This  is^an  image  formed  by  the  actual 
meeting  of  rays)  such  images  can  always  be  projected  on 
to  a  screen. 

A  Virtual  Image. — This  is  one  that  is  formed  by  the 
prolongation  backward  of  rays  of  light  to  a  point. 


FIG.  42. 

/To  find  the  position  and  size  of  an  image  it  is  necegsary 
to  obtain  the  conjugate  foci  of  the  extremes  of  the  object, 
as  the  image  of  an  object  is  equal  to  the  sum  of  its  inter- 
mediate points.  iOnly  two  rays  are  required  for  this  pUr- 
pose,  one  parallel  to  the  axial  ray,  and  one  secondary  ray 
passing  through  the  optic  center  ;  the  image  of  the  extreme 
point  of  the  object  will  be  located  at  the  point  of  inter- 
section of  these  rays.  In  figure  42  A  B  is  an  object  in 
front  of  a  convex  lens,  o  is  the  optic  center  and  P.  F. 
the  principal  focus.  A  ray  drawn  from  A  parallel  to  the 
axial  ray  o,  and  a  secondary  ray  from  the  same  point  drawn 


OPTICS. 

through  the  optic  center,  will  give  at  their  point  of  inter- 
section the  conjugate  focus  of  the  luminous  point  A,  which 
will  be  at  A'.  In  the  same  way  the  conjugate  focus  of  B 
and  points  intermediate  in  the  object  may  be  obtained.  A' 
B'  is  a  real  inverted  image  of  A  B  ;  /the  size  of  the  image 
of  A  B  depends  upon  the  distance  of  the  object  from 
the  lens.  The  relative  sizes  of  image  and  object  are  as  , 
their  respective  distances  from  the  optic  center  of  the  lens.) 
For  example,  if  an  object  ten  millimeters  high  is  three 
meters  (3000  mm.)  from  the  optic  center  of  a  lens,  and  its 
image  is  sixty  millimeters  from  the  lens,  the  image  will  be 

ToMhr  or  To"  °f  tne  s'ze  °f  tne  object  ;  that  is,  the  image  will 
be  -^  of  ten  millimeters  (the  height  of  the  object)  —  namely, 
I  of  a  millimeter  high. 

As  conjugate  foci  are  interchangeable,  then  in  figure  42 
if  A!  B'  was  the  object,  the  image  A  B  would  be  the  image 
of  A'  B',  and,  therefore,  larger  than  the  object. 

Three  facts  should  be  borne  in  mind  in  the  study  of  real 
images  formed  by  a  convex  Inis  :  ^~\^JjuC  \ 

1.  The  object  and  image  are  interchangeable. 

2.  The'object  and  the  real  image  are  on  opposite  sides  of    "T) 

the  lens,  and, 

0>    ^  / 

3.  As  the  rays  which    pass    through  the    optic    center 

cross  each  other  at  this  point,  the  real  image  must  be  in- 
verted.  A&*r«uS\>W^ 

^MMH^^M*  '  *f  L3 

Rays  of  light  from  an  object  situated  at  the  distance  of    Q^  J  / 


the  principal  focus  would  proceed  parallel  after  refraction,    r~  r>1-t' 

•P- 


and  no  image  of  the  object  would  be  obtained. 


/  If  an  object  is  situated  just  beyond  the  principal  focus, 
/  then  the  image  would  be  larger  than  the  object,  real  and 
/  inverted.  (See  Fig.  42,  reversing  image  for  object.) 

£  If  an  object  is  situated  at  twice__the  distance  of  the  prin- 

cipal focus,  then  its  image  would  be  of  the  same  size,  real, 


4O         REFRACTION  AND  HOW  TO  REFRACT. 

inverted,  and  at  a  corresponding  distance,  as  these  conju- 
gate foci  are  equal.y 

If  an  object  is  situated  at  a  greater  distance  than  twice 
the  principal  focus,  and  nearer  than  infinity,  its  image  will 
be  real,  inverted,  and  smaller  than  the  object. 


FIG.  43. 

Rays  of  light  from  an  object  situated  closer  to  a  lens 
than  its  principal  focus  would  be  divergent  after  refraction, 
and  could  only  meet  by  being  projected  backward  ;  the 
image  would,  therefore,  be  larger  than  the  object,  erect,  and 


FIG.  44. 

virtual.  Such  an  image  is  only  seen  by  looking  through 
the  lens ;  the  lens  in  this  instance  being  a  magnifying 
glass.  (Fig.  43.) 

Images  Formed  by  Concave  Lenses. — These  images 
are  always  erect,  virtual,  and  smaller  than  the  object.  (See 
Fig.  44.)  A  concave  lens  is,  therefore,  a  minifying  lens. 

&AC& 


OPTICS.  41 

Parallel  rays  from  the  extremes  of  the  object  A  ix  form  the 
divergent  ray  A'  and  R'  after  refraction.  Secondary  rays 
pass  through  the  optic  center  o  unrefracted,  A"  and  R". 
At  the  points  of  intersection  where  these  rays  meet  after 
being  projected  backward,  the  image  of  A  R  is  found, 
erect,  virtual,  and  diminished  in  size.  This  image  is  only 
;een  by  looking  through  the  lens. 

Numeration  of  Lenses. -/-Formerly,  lenses  were  num- 
bered according  to  their  radii  of  curvature  in  Paris  inches 
(27.07  mm.).  The  unit  was  a  lens  that  focused  parallel 
rays  of  light  at  the  distance  of  one  English  inch  (25.4  mm.) 
from  its  optic  center,/ 

As  lenses  for  purposes  of  refraction  were  never  as  strong 
as  the  unit,  they  were  numbered  by  fractions,  thus  showing 
their  relative  strength  as  compared  to  this  unit ;  for  instance, 
a  lens  that  was  one-fourth  the  strength  of  the  unit  was 
expressed  by  the  fraction  ^,  or  a  lens  that  was  one- 
sixteenth  the  strength  of  the  unit  was  expressed  as  ^g-,  etc., 
[the  denominator  of  the  fraction  indicating  the  focal  length 
'of  the  lens  in  Paris  inches. 

There  are  three  objections  to  this  nomenclature  :  (i) 
The  difference  in  length  of  the  inch  in  different  countries  ; 
(2)  the  inconvenience  of  adding  two  or  more  lenses  num- 
bered in  fractions  with  different  denominators —  ylj  -f  -£% 
-f-  ^;  (3)  the  want  of  uniform  intervals  between  num- 
bers. 

In  the  new  nomenclature,  and  the  one  that  is  now  quite 
universal,  known  as  the  metric  or  dioptric  system  (diopter, 
abbreviated  D.),/^f  lens  has  been  taken  as  the  unit  which 
has  its  principal  focus  at  one  meter  distance  (39.37  English 
inches),  commonly  recognized  as  40  inches.  c.">  /  rv 

^Lenses  in  the  dioptric  system  are  numbered  according 
to  their  refractive  power  and  not  according  to  their  radii  of 


42         REFRACTION  AND  HOW  TO  REFRACT. 

curvature.") (The  strength  or  refractive  power  of  a  dioptric 
lens  is,  tKerefore,  the  inverse  of  its  focal  distance.)  To  find 
jthe  focal  distance  of  any  dioptric  lens  in  inches  or  centi- 
Imeters,  the  number  of  diopters  expressed  must  be  divided 
linto  the  unit  of  40  inches  or  100  cm.  For  example,  a 
2  D.  lens  has  a  focal  distance  of  40  -4-  2  equals  20  inches  ; 
or  100  cm.  -4-  2  equals  50  cm.  A  -(-4  D.  has  a  focal  dis- 
tance of  40  -4-  4,  equaling  10  inches,  or  100  -4-  4,  equaling  25 
cm.  /Lenses  that  have  a  refractive  power  less  than  the  unit 
are  not  expressed  in  the  form  of  fractions,  but  in  the  form  of 
decimals  ;  for  example,  a  lens  which  is  only  one-fourth,  one- 
half,  or  three-fourths  the  strength  of  the  unit  is  written  0.25, 
0.50,  0.75,  respectively,  and  their  focal  distances  are  found 
in  the  same  way  as  in  dealing  with  unitsj)  0.25  D.  has  a 
focal  distance  of  40  H-  0.25  or  100  -4-  0.25,  equaling  160 
inches  or  400  cm.  ;  0.50  D.  has  a  focal  length  of  40  -4- 
0.50  or  100  -4-  0.5-0,  equaling  80  inches  or  200  cm.  ;  0.75 
D.  has  a  focal  length  of  40  -4-  0.75  or  100  -4-  0.75 
equaling  53  inches  or  133  cm.  Unfortunately,  0.25  D., 
0.50  D.,  and  0.75  D.  are  frequently  spoken  of  as  twenty- 
five,  fifty,  and  seventy-five,  which  occasionally  leads  to 
confusion  in  the  consideration  of  the  strength  and  focal  dis- 
tance. The  student  should  learn  as  soon  as  possible  to 
change  the  old  nomenclature  into  the  new,  as  he  will  have 
to  make  these  changes  in  reading  other  text-books. 

To  change  the  old  "  focal  length  "  or  inch  system  of 
numbering  lenses  into  diopters,  divide  the  unit  (40  in.)  by 
the  denominator  of  the  fraction,  and  the  result  will  be  an 
approximation  in  diopters  ;/  for  example,  y^-  equals  y$ 
or  4  D.  ;  -^V  equals  |^  or  2"  D.  The  following  table, 
from  Landolt,  gives  the  equivalents  in  the  old  and  new 
systems : 


OPTIQS. 


43 


OLD  SYSTEM. 

NEW  SYSTEM. 

I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

No. 

No. 

No. 

of  the 

Focal 

Focal 

of  the 

Focal 

Focal 

Cories- 

Lens, 

Distance 

Distance 

Equiva- 

Lens, 

Distance 

Distance 

poiKlinir 

Old 

in  English 

in  Milli- 

lent in 

New 

in  Milli- 

in English 

of  the  Old 

System. 

Inches. 

meters. 

Diopters. 

System. 

meters. 

Indies. 

System. 

72 

67.9 

1724 

0.58 

0.25 

4000 

IS7  48 

166.94 

60 

56.6 

'437 

0.695 

'    0.5 

2OOO 

7874 

8346 

48 

45-3 

1150 

0.87 

o.75 

1333 

52-5 

5563 

42 

39-6 

1005 

0.99 

i 

1000 

3937 

41-73 

36 

34 

863 

I  16 

'•25 

800 

3i  5 

3339 

3« 

28.3 

718 

i  39 

i-5 

666 

26.22 

2779 

24 

22.6 

574 

1-74 

i-75 

571 

22.48 

23-83 

20 

18.8 

477 

2.09 

2 

500 

19.69 

20.87 

18 

I? 

431 

2-31 

2.25 

444 

1748 

1853 

16 

15 

38i 

2.6 

2-5 

400 

15-75 

16.69 

15 

I4.I 

358 

2.79 

3 

333 

"3  i? 

"3-9 

14 

13-2 

335 

2.98 

3-5 

286 

ii  26 

n-94 

13 

12.2 

312 

3-20 

4 

250 

9.84 

10.43 

12 

II.  2 

287 

3-48 

45 

222 

8-74 

9  26 

II 

I°3 

261 

3.82 

5 

2OO 

7.87 

8-35 

10 

9-4 

239 

4.18 

5-5 

182 

7  16 

76 

9 

8-5 

216 

4.63 

6 

166 

654 

693 

8 

7-5 

190 

5-2.S 

7 

'43 

563 

597 

7 

6.6 

167 

5-96 

8 

125 

4.92 

5-22 

(>K 

6.13 

155 

642 

9 

III 

4-37 

4-63 

6 

5-6 

142 

70 

10 

IOO 

394 

4  '7 

5% 

52 

132 

7-57 

ii 

91 

3-58 

38 

5  f 

4-7 

119 

8.4 

12 

83 

3-27 

3-46 

4% 

42 

106 

9-4 

13 

77 

3-°3 

3-21 

4  f 

3-8 

96 

10.4 

14 

7i 

2.8 

2.96 

3% 

3-3 

84 

11.9 

15 

67 

2.64 

2.8 

3Y4 

3-1 

79 

12.7 

16 

62 

244 

2.59 

3 

2.8 

71 

14.0 

17 

59 

2.32 

2  46 

2K 

2.6 

66 

15-1 

18 

55 

2  17 

2.29 

*S 

2.36 

60 

17.7 

20 

5° 

1.97 

2.09 

2* 

2.1 

53 

18.7 

2 

1.88 

48 

20.94 

Cylindric  Lenses. — Abbreviated  cyl.,  c,  or  C.  A  cylin- 
dric  lens,  usually  called  a  "  cylinder,"  receives  its  name  from 
being  a  segment  of  a  cylinder  parallel  to  its  axis.  (See 
Fig.  45. T  Occasionally  cylinders  are  made  with  both  sur- 
faces curved,  and  are  then  equivalent  to  two  planocylinders 
with  their  plane  surfaces  together.  A  cylinder  may  be 
defined  as  a  lens  u'hich  refracts  rays  of  light  opposite  to  its 
axis.  This  definition  should  be  carefully  borne  in  mind  in 
contradistinction  to  a  spheric  lens,  which  refracts  rays  of 
light  equally  in  all  meridians.  /A  cylindric  lens  has  no  one 


44 


REFRACTION  AND  HOW  TO  REFRACT. 


common   focus  or  focal  point,  but  a  line  of  foci,  which  is 
parallel  to  its  axis.) 

Axis  of  a  Cylinder. — That  dimension  of  a  cylindric  lens 
which  is   parallel   to   the  axis  of  the  original  cylinder  of' 


FIG.  45. 


FIG.  46. 


FIG.  47- 


which  it  is  a  part  is  spoken  of  as  the  axis,  and  is  indicated 
on  the  lens  of  the  trial-case  by  a  short  diamond  scratch  on 
the  lens  at  its  periphery,  or  by  having  a  small  portion  of 


FIG.  48. 


its  surface  corresponding  to  the  axis  ground  at  the  edges, 
or  it  may  be  marked  in  both  ways.     (See  Fig.  47.)     Cylin- 
ders are  of  two  kinds — convex  and  concave.    (Figs.  45,  46.) 
Cylinder  Action.— /A  convex  cylinder  converges  parallel 


OPTICS. 


45 


FIG.  49. 


rays  of  light  so  that  after  refraction  they  are  brought  into 
a  straight  line  which  corresponds  to  the  axis  of  the  cylin- 
der j/for  instance,  a  +5  cyl.  will  converge  parallel  rays  so 
that  they  come  together  in  a  straight  line  at  the  dis- 
tance of  eight  inches, 
or  twenty  centimeters, 
and  this  straight  line 
will  be  parallel  to  the 
axis  of  the  cylinder. 
(Fig.  48.) 

A  concave  cylin- 
der diverges  rays  of 
light  opposite  to  its 

axis,  as  if  they  had  diverged  from  a  straight  line  on  the 
opposite  side  of  the  lens.      (Fig.  49.) 

Spherocylinders. — A  spherocylinder  is  a  combination 
of  a  sphere  and  a  cylinder,  and  is/Therefore  a  lens  which  has 
one  surface  ground  with  a  spheric  curve  and  the  other  sur- 
face cylindric/  A  spherocylindric  lens  is/also  spoken  of  as 
an  astigmatic  lensj1  (See  Fig.  100.)  A  spnerocylindric  lens 
is{pne  which  has  two  focal  planes.j  Spherocylinders  have 
different  curves  :  the  spheric  curve  may  be  convex,  with  the  ! 
cylindric  surface  convex  ;  or  the  spheric  surface  may  be 
concave,  with  the  cylindric  surface  concave  ;  or  the  spheric 
surface  may  be  convex,  with  the  cylindric  surface  concave  ; 
or  the  spheric  surface  may  be  concave,  with  the  cylindric 
surface  convex. 

The  Trial-case  (see  Fig.  50).  — This  case  contains  pairs 
of  plus  and  minus  spheres  and  pairs  of  plus  and  minus 
cylinders  ;  also  prisms  numbered  from  ^  or  ^  to  20  A. 
The  spheres  are  numbered  in  intervals  of  o.  12  up  to  2  S.; 
and  from  2  S.  up  to  5  S.  the  interval  is  0.25  S.;  and  from 
5  S.  to  8  S.  the  interval  is  0.50  S.;  and  from  8  S.  to  22  S. 


46         REFRACTION  AND  HOW  TO  REFRACT. 

the  interval  is  i  S.     The  /cylinders  have  similar  intervals, 
but  seldom  go  higher  than  6  or  8  cyy 

The  trial-case  also  contains  a  trial -frame,  which  is  used 
to  place  lenses  in  front  of  the  patient's  eyes.  (See  Fig.  51.) 
The  eye -pieces  of  such  a  frame  are  numbered  on  the 
periphery  in  degrees  of  half  a  circle,  so  that  the  axis  of  a 
cylinder  can  be  seen  during  refraction.  The  left  of  the 


FIG.  50. 

horizontal  line  in  each  eye-piece  is  recognized  as  the  start- 
ing-place, or  zero  (o),  and  the  degrees  are  marked  from 
left  to  right  on  the  lower  half,  counting  around  to  the 
horizontal  meridian,  which  at  the  right  hand  is  numbered 
1 80;  this  horizontal  meridian  is,  therefore,  spoken  of  as 
horizontal,  zero  (O),  or  1 80  degrees.  The  meridian  midway 
between  zero  and  1 80  is  spoken  of  as  vertical,  or  90  degrees. 
In  some  countries  the  meridians  are  differently  num- 


OPTICS. 


47 


bered  (see  Fig.  52);  for  example,  the  vertical  meridian  is 
called  zero,  and  the  degrees  are  marked  on  each  side  of 
zero  up  to  90  degrees.  Only  the  upper  half  of  the  eye-piece 
is  thus  numbered,  so  that  when  a  cylinder  has  the  upper  end 
of  its  axis  inclined  toward  the  nose,  the  record  would  be  so 
many  degrees  of  inclination  to  the  nasal  side  ;  or  if  the  upper 
end  of  the  cylinder  was  inclined  toward  the  temple,  the 
record  would  be  so  many  degrees  to  the  temporal  side. 
For  example,  in  the  right  eye  1 5  degrees  nasal  would 


mean  axis  75  on  the  ordinary  trial-frame,  and  15  degrees 
temporal  would  mean  105  degrees. 

The  trial-case  also  contains  other  accessories,  such  as 
blanks  or  blinders,  a  stenopeic  slit,  pin-hole  disc,  etc.,  all  of 
which  are  referred  to  in  the  text. 

Combination  of  Lenses. — The  sign  of  combination 
is  ^. 

Combining  Spheres. — Any  number  of  spheric  lenses 
placed  with  their  optic  centers  over  each  other,  and  sur- 


48 


REFRACTION  AND  HOW  TO  REFRACT. 


faces  together,  will  equal  one  lens  the  value  of  their  sum  : 
for  example,  -f-2  S.  Q  -f  I  S.  Q  -(-3  S.  will  equal  -j-6  S.; 
or  a  — 2  S.  ^  —  I  S.  (^  — 3  S.  will  equal  a  — 6  S. 

If  a  plus  and  minus  sphere,  each  of  the  same  strength,  be 
placed  with  their  optic  centers  together,  the  refraction  will 
be  nothing,  for  the  one  will  neutralize  the  effect  of  the  other  ; 
for  instance,  +48.  and  — 4  S.  will  be  equivalent  to  a  piece 
of  plane  glass,  as  the  — 4  S.  will  diverge  rays  of  light  as 
much  as  the  -(-4  S.  will  converge  them,  and  the  result 


FIG.  52. 

is,  rays  of  light  parallel  before  refraction  are  parallel  after 
passing  through  such  a  combination.  (If,  however,  a  plus 
and  a  minus  sphere  of  different  strengths  are  placed  together, 
the  value  of  the  resulting  lens  will  equal  their  difference,  in 
favor  of  the  higher  numberj  for  instance,  -{-48.  and  — 2 
S.  will  equal  a  -\-2  S.,  the  — 2  S.  neutralizing  2  S.  of  the 
+48.,  leaving +2  S. 

Combining  Cylindric  Lenses. -(-Any  number  of  cylin- 
dric  lenses  placed  together,  with  their  axes  in  the  same 


OPTICS.  49 

meridian,  are  equal  to  a  cylinder  the  value  of  their  sum ; 
for  example  :  -j-2  cyl.  axis  90  degrees  and  +3  cyl.  axis  90 
degrees  will  equal  a  -f  5  cyl.  axis  90  degrees  ;  or  — 2  cyl. 
axis  1 80  degrees  and  — 3  cyl.  axis  180  degrees  will  equal 
a  — 5  cyl.  axis  180  degrees  ;  or  — 2  cyl.  axis  180  degrees 
and  +i  cyl.  axis  180  degrees  will  equal  a — I  cyl.  axis 
1 80  degrees. 

As  a  cylinder  refracts  rays  of  light  only  in  the  meridian 
opposite  to  its  axis,  this  opposite  meridian  can  always  be 
found  by  the  following  simple  rule  : 

"  Add  90  u'hcn  the  given  axis  is  yo  or  less  than  po,  and 
subtract  go  when  the  given  axis  is  more  than  90."  j 

For  example :  -f-  3  cyl.  axis  90  refracts  rays  of  light  in 
the  1 80  degree  meridian  (90  +  90=  180);  or  +3  cyl. 
axis  75  refracts  rays  of  light  in  the  165  meridian  (75+90 

—  165).     A  — 3  cyl.  axis  135  refracts  rays  of  light  in  the 
45  meridian  (135  less  90  =45).      A  — 2  cyl.  axis  180  re- 
fracts rays  of  light  in  the  90  meridian  (180  less  90  =  90). 

Combining  two  cylinders  of  the  same  strength  and 
same  denomination,  with  their  axes  at  right  angles  to 
each  other,  will  equal  a  sphere  of  the  same  strength 
and  same  denomination.  For  instance,  +3  cyl.  axis  90 
and  +3  cyl.  axis  180,  placed  together,  will  equal  a  +3  S. 

—  /.  e.y  the  +3  cyl.  at  axis  90  will  converge  parallel  rays  in 
the  1 80  meridian,  while  the  -f-  3  cyl.  axis  1 80  will  converge 
parallel  rays    in    the    90    meridian,  producing  a  principal 
focus /therefore  any  sphere  is  also  equal  to  two  cylinders 
of  its    same    strength  and  same  denomination  with  their 
axes  at  right  angles  to  each  otherj 

(Combining  cylinders  of  different  strength,  but  of  the 
same  denomination,  with  their  axes  at  right  angles  to 
each  other,  such  a  combination  will  equal  a  sphere  and 
a  cylinder  of  the  same  denomination.  )  For  example : 


REFRACTION  AND  HOW  TO  REFRACT. 


-\-2  cyl.  axis  75  O  +3  cyl.  axis  165  will  equal  -f-2  S.  O 
~\-i  cyl.  axis  165.  The  -\-2  cyl.  axis  75  takes  -f  2  of  the 
-j~3  cyl.  axis  165  and  makes  a  +2  S.,  leaving  -f- 1  cyl.  axis 
at  165  ;  the  result  is  then  -f-2  S.  O  +  I  cyl.  axis  165. 

Or  — 3-5O  cyl.  axis  15  O  - — 4.50  cyl.  axis  105,  will 
equal  — 3.50  S.  O  — I  cyl.  axis  105.  The  — 3.50  axis  15 
takes  —3.50  of  the  — 4.50  and  makes  a  — 3.50  S., 
leaving  — I  cyl.  axis  105;  this  — I  cyl.  axis  105  is  now 
joined  to  the  — 3-5O  sphere,  making  — 3-5O  S.  O  — I  cyl. 
axis  105. 

Combining  a  sphere   and   a  cylinder  of  the   same 

strength,  but  of  differ- 
ent denomination,  will 
'equal  a  cylinder  of  the 
opposite  sign  and  opposite 
axis  from  the  cylinder 
given.  For  example  :  -f- 1 
sphere  O  —  I  cyl.  axis  1 80 
will  equal  -f-  I  cyl.  axis  90. 
The  -f-  i  S.  equals  two  -f-  I 
cylinders,  one  at  axis  90 
and  one  at  axis  180,  and 
the  — I  cyl.  at  axis  180  is 
neutralized  by  the  -f- 1  cyl. 
at  the  same  axis,  leaving 

-f-  I  cyl.  axis  90.    This  may  be  better  understood  by  the 
diagram  (Fig.  -49). 

Or  —3  S.  O  +3  cyl.  axis  90  equals  — 3  cyl.  axis  180. 
The  — 3  S.  is  equal  to  two  — 3  cylinders,  one  at  axis  90 
and  one  at  axis  180 ;  the  one  at  axis  90  is  neutralized  by 
the  +3  cyl.  at  the  same  axis,  leaving  — 3  cyl.  axis  180. 

Combining  a  Sphere  with  a  Weaker  Cylinder  of  Dif- 
ferent Denomination. — Such  a  combination  should  be 


1.00 


FIG.  53. 


OPTICS.  5 1 

changed  to  its  simplest  form  of  expression,  and  will  equal 
a  sphere  of  the  same  denomination,  of  the  value  of  their 
difference,  combined  with  a  cylinder  of  the  same  strength 
as  the  cylinder  given,  but  of  opposite  sign  and  axis.  For 
example:  -j~4  S.  O — i  cyl.  axis  180.  The  minus  one 
cylinder  is  refracting  in  the  90  degree  meridian,  therefore  it 
reduces  the  strength  of  the  +4  S.  in  this  axis,  making  it  a 
plus  3.  The  horizontal  or  1 80  degree  meridian  of  the  plus 
4  S.  has  not  been  altered,  but  still  remains  -(-4,  and  the  re- 
sult is,  plus  3  in  the  vertical  meridian  and  plus  4  in  the 
horizontal  meridian,  equaling,  therefore,  -j~3  S.  ^  -f  I 
cyl.  axis  go. 

The  following  rule  will  be  of  service  in    making    this 
change,  and,  in  fact,  this  rule  will   apply   in  any  instance 

where  the  sphere  and  cylinder  are  of  different  denomina- 

/s^ 
tion,  no  matter  what  their  respective  strengths  may  be  :       |(/^ 

Rule. — Subtract  the  less  from  the  greater,  and  to  the  res2tlh.    { 
prefix  the  sign  of  the  greater ;  continue  isith  this  the  same\     ^',(Y^ 
strength  cylinder,  using  the  opposite  sign  and  opposite  axis.\  ^T^*  \ 
Example  :   +2.25  S.  O  — 0.75  cyl.  axis  75  degrees  ;  sub- •/**' 
tracting  the  less  from  the  greater  ( — 0.75  from  +2.25),  and\ 
prefixing  the  sign  of  the  greater  (+),  will  leave  -}- 1.50  S.  ; 
and  combining  with  this  the  same  strength  cylinder  (0.75), 
with  opposite  sign  and  axis  (+  and  165),  will  be  +0.75 
cyl.  axis  165.      Result,  +I-5O  S.  O  +0.75  cyl.  axis   165. 
Combining  a  Sphere  and  Cylinder  of  the  Same  De- 
nomination. —  This    is    recognized    as    the    minimum    or 
^  simplest  form  of  expression,  and  is  seldom  changed.      For 
example  :  — 2  S.  O  — 6  cyl.  axis  180  is  considered  as  the 
thinnest  lens  and  the  one  with  the  least  weight  that  can  be 
made  by  such  a  combination.      It  may  be  changed,  how- 
ever, by  the  reverse  of  the  rule  above  given,  and  will  equal 
— 8  S.  O  -f  6  cyl.  axis  90. 


REFRACTION  AND  HOW  TO  REFRACT. 

Combining  Two  Cylinders  of  Different  Denominations 
with  Opposite  Axes. — Commonly  called  crossed  cylinders. 
Such  combinations  can  be  written  in  three  ways  : 

1.  -)-Cyl.  ^ — cyl.  axes  opposite. 

2.  -f  Sphere  O  — cyl.  (cylinder  stronger  than  sphere). 

3.  — Sphere  +cyl.  (cylinder  stronger  than  sphere). 

y     For  example  :  — i.oo  cyl.  axis  180^  +2.50  cyl.  axis  90 
may  be  changed  to  one  of  the  following  : 

— I.oo  S.  O  +3-5°  cyl.  axis  90  ;  or — 
^+2.50  S.  C  —3-50  cyl.  axis  180.' 

\J|    '     \      \      i7,   L^   t*»T. 
•  /K/Y^   n~V^«^Jt^" 

-j  The  first  formula  shows  that  the  vertical  meridian  must 
always  be  — I  and  the  horizontal  or  1 80  meridian  must 
always  be  +2.50,  and  with  this  clearly  in  mind,  the  second 
/^?4rfkf  '&ird  formulas  will  be  understood.  In  the  second 
&**  '  formula  ( — i.oo  S.  O  +3.50  cyl.  axis  90)  the  +3.50  cyl. 
is  only  equal  to  +2.50,  as  it  has  I  D.  neutralized  by  — I 
of  the  — I  sphere.  In  the  third  formula  (+2.50  S.  O 
»  "  ^^/3/5°  cyl.  axis  !8o)  the  — 3-5°  cylinder  is  only  equal  to 
— HT.OO  cylinder,  as  it  has  — 2.50  neutralized  by  +2.50  of 
the  sphere. 

In  any  spherocylindric  combination  the  meridian  in  which 
the  axis  of  the  cylinder  lies  has  the  strength  of  one  lens,  and 
the  meridian  opposite  to  the  axis  of  the  cylinder  has  the  com- 
bined values  of  sphere  and  cylinder — /.  c.,  — i.oo  S.  O 
-(-3.50  cyl.  axis  90  means  — i.oo  on  the  axis  (90)  of  the 
cylinder,  and  opposite  to  the  axis  therefore  at  1 80,  it  equals 
+  2.50  (—1  and  4-3-50). 

Crossed  cylinders  in  themselves  are  seldom  ordered  in  a 
prescription,  preference  being  given  to  a  spherocylindric 
combination,  ('when  to  order  a  plus  sphere  with  a  minus 
cylinder,  and  when  to  order  a  minus  sphere  with  a  plus 
cylinder,  depends  upon  the  individual  lenses.)  For  example  : 


-(-0.50  cyl.  axis  90  O  — 5.00  cyl.  axis  180  equals  +0.50 
S.  O  — 5.50  cyl.  axis  iSoor  — 5  S.  O  +5. 50  cyl.  axis  90.' 

Preference  would  be  given  to  the  plus  sphere  combina- 
tion, on  account  of  thinness  and  lesser  weight  of  the  lens. 
The  following  formula,  — i  cyl.  axis  180  degrees  ^  —3 
cyl.  axis  90  degrees,  equals  — i.oo  S.  ^  +4  cyl.  axis  90, 
or  +3  S.  ^  — 4  cyl.  axis  180,  and  for  similar  reasons 
preference  would  be  given  to  the  minus  sphere  combination. 
Whichever  combination  makes  the  thinnest  and  lightest 
weight  of  glass  is  the  one  to  be  ordered,  as  a  rule. 

The  student  should  practise  these  combinations  at  the 
trial-case,  and  be  able  at  a  glance  to  change  one  formula 
into  another  without  diagram  or  rule. 

Prescription  Writing. — In  writing  prescriptions  for 
lenses  the  right  eye  is  indicated  by  one  of  three  signs — R, 
Rt,  or  O.  D.,  the  latter  from  the  Latin  for  right  eye, 
OCH/HS  Dexter.  The  left  eye  is  also  indicated  in  one  of 
three  ways — L,  Lt,  or  O.  S.,  the  latter  from  the  Latin 
for  left  eye,  Oculus  Sinister. 

A  prescription  may  call  for  any  one  of  the  following  : 

-(-Sphere,  written  4  4  D.  or  -(-4.00  D.  S.  or  -f  4  S.  or  -}  4  sph. 
— Sphere,  written  — 2  D.  or  — 2.00  D.  S.  or  — 2  S.  or  — 2  sph. 
-(-Cylinder,  written  -f  4.00  D.  C.  or  -f  4  C.  or  -(-4  cyl.  (axis  as  indicated). 
— Cylinder,  written  — 2.00  D.  C.  or  — 2  C.  or  — 2  cyl.  (axis  as  indicated), 
-f  Sphere  and  -f-  cylinder,  written  +2.00  S.  ^  -\-2.oo  cyl.  axis  90  degrees. 
— Sphere  and  — cylinder,  written  — 2.00  S.  ^ — 2.00  cyl.  axis  180  degrees. 
-f  Sphere  and  — cylinder  (cylinder  stronger  than  sphere),  42-°°  S.  O — 3-OO 

cyl.  axis  180  degrees.  C 

— Sphere  and  -f  cylinder  (cylinder  stronger  than  sphere),  — 2.00  S.  ^  43-°°  ) 

cyl.  axis  90  degrees. 

A  plus  cylinder  and  minus  cylinder  may  be  prescribed, 
and,  if  so,  their  axes  must  be  at  right  angles  to  each  other. 
An  occasional  exception  to  this  may  be  found  in  irregular 
astigmatism.  Or  a  prism  with  its  base  indicated  may  be 


^ 


REFRACTION  AND  HOW  TO  REFRACT. 


&! 

\j  "Nadded  in  any  one  of  the  foregoing  formulas  ;  for  example  : 
*  — 2  S.  O  — 2.OO  cyl.  axis  1 80  O  2  A  base  in  ;  or  the  direc- 
>tion  of  the  base   may  be    abbreviated   as  follows  :    B.   I., 
^  meaning  base  in  ;  B.  O.,  meaning  base  out ;  B.  U.,  meaning 
base  up ;  and  B.  D.,  meaning  base  down. 

Prescriptions  are  never  written  for  two  spheres. 
Prescriptions  are  never  written  for  two  cylinders  at  the 
ame  axis. 

Prescriptions  are  never  written  for  two  cylinders  at  axes 
Bother  than  those  at  right  angles  to  each   other,  except,  as 
just  noted,  in  irregular  astigmatism. 

For  obvious  reasons  prescriptions  are  never  written  for  a 
sphere  and  two  cylinders^  except  in  irregular  astigmatism. 
Recognition  of  Lenses? 

A  convex  sphere  is  thick  at  the  center  and  thin  at  the 
r\<r   1   edge.     It  has  the  power  of  converging  rays  of  light ;  hence, 
L  \    V^f  strong,  it  is  a  burning  glass.     Objects  viewed  through  a 
i   Q  convex  lens  as  it  is  moved  before  the  eye,  from  left   to 
r  right  and  right  to  left  or  up  and  down,  appear  to   move 
^  in  an    opposite    direction    to    that   in    which    the    lens    is 
5V'    v  moved.       The    weaker   the    lens,    the    slower   the    object 
~~%  (appears  to  move ;  and  the  stronger  the  lens,  the  faster  the 
V  apparent  movement  of  the  object.  (A  convex  lens  being  a 
V  magnifier,  has  the  effect  of  making  objects  appear  larger 
)  and  closer  when  it  is  moved  away  from  the  observer's  eye  ; 
•   v,or  if  brought    toward    the  eye,  objects    already  enlarged  V 
viappear  smaller  and  more  distant^ 

A  To  Find  the  Optic  Center  of  a  Convex  Lens. — Look- 
^ing  at  a  vertical  straight  line  and  passing  a  convex 
lens  before  the  eye  from  left  to  right  has  the  effect  of 
displacing 'to  ward  thff^right  edge  of  the  lens  that  portion  of' 
the  line  seeir\  through  the  lens  (see  , Fig.  54),  and  as  the 
lens  is  slowlV  moyed^$till  further  to  th&  right,  the  displacec 


1 


OPTICS. 


55 


portion  of  the  line  will  finally  coincide  with  the  original 
straight  line,  making  one  continuous  line  through  the  lens. 
(See  Fig.  55.)  Marking  this  straight 
line  on  the  surface  of  the  lens,  and 
then  turning  the  lens  to  the  opposite 
meridian  and  repeating  the  examina- 
tion, and  marking  the  lens  as  before, 
the  optic  center  will  be  in  the  lens 
beneath  the  point  of  intersection  of  the 
t\vo  lines.  (See  Fig.  56.) 

A  concave  sphere  is  thick  at  the 
edge  and  thin  at  the  center,  and  has 
the  power  of  causing  rays  of  light 
to  diverge.  When  moved  before  the 
eye  from  left  to  right  and  right  to  left 
or  up  and  down,  objects  appear  to  move  in  the  same  direc- 
tion as  that  in  which  the  lensjs  moved. 

A  concave  lens  being  a  minifier,  makes  objects  appear 


FIG.  55. 


FIG.  56. 


smaller  and  more  distant  as  the  glass  is  moved  away  from 
the  eye,  and  if  brought  closer  to  the  eye,  makes  objects 
apparently  small  appear  somewhat  larger  and  nearer. 


REFRACTION  AND  HOW  TO  REFRACT. 


Looking  at  a  straight  edge  or  line  through  a  concave 
sphere,  and  passing  the  lens  from  left  to  right,  the  portion 
of  the(line  seen  through  the  lens  appears  displaced  toward 
the  center  of  the  lens  (see  Fig.  57),  and  as  the  lens  is  still 
further  moved  to  the  right,  the  displaced  portion  of  the 
line  finally  coincides  with  the  original  straight  edge,  as  in 
figure  55.) 

The  optic  center  of  a  concave  lens  is  found  in  the  same 
way  as  the  center  of  a  convex  lens. 

A  Convex  Cylinder. — When  a  convex  cylinder  is  moved 


FIG.  57. 


FIG.  58. 


in  front  of  the  eye  in  the  direction  of  its  axis,  objects  looked 
at  do  not  change  their  positions  ;  but  when  the  lens  is 
moved  in  the  direction  opposite  to  its  axjs;  the  movement 
of  the  object  is  the  same  as  that  of  a  convex  sphere.  Look- 
ing at  a  straight  edge  through  a  convex  cylinder,  and 
rotating  it,  has  the  effect  of  displacing  away  from  its  axis 
that  portion  of  the  straight  edge  seen  through  the  lens. 
(See  Fig.  58.) 

A   Concave  Cylinder. — When   a   concave   cylinder   is 
moved  in  front  of  the  eye  in  the  direction  of  its  axis,  ob- 


OPTICS.  57 

jects  looked  at  do  not  change  their  positions  ;  but  when  the 
lens  is  moved  in  the  direction  opposite  to  its  axis,  the 
movement  of  the  object  is  the  same  as  that  of  a  concave 
sphere.  Looking  at  a  straight  line  through  a  concave 
cylinder,  and  rotating  it,  has  the  effect  of  displacing  toward 
its  axis  that  portion  of  the  straight  line  seen  through  the 
lens.  (See  Fig.  59.)  /  A  circle  viewed  through  a  strong  con- 
cave cylinder  appears  as  an  oval  with  its  long  diameter  cor- 
responding to  its  axis.^)  (See  Fig.  60.)  (A  circle  viewed 
through  a  strong  convex  cylinder  appears  as  an  oval  with 
its  long  diameter  opposite  to  its  axislN  In  place  of  using  a 


FIG.  59.  FIG.  60. 


straight  line  or  straight  edge  to  find  the  optic  center  of  a 
sphere  or  axis  of  a  cylinder,  two  lines  at  right  angles  may 
be  substituted  (see  Fig.  56)  or  a  protractor  may  be  used. 

A  Prism. — Objects  viewed  through  a  prism  are  dis- 
placed toward  its  apex,  and  that  portion  of  a  straight  line 
seen  through  the  prism  never  coincides  with  the  straight 
line. 

Neutralization  of  Lenses. — Having  determined  from 
the  foregoing  description  what  the  character  of  an  indi- 
vidual lens  may  be,  then  to  neutralize  its  effect  or  find  out 
its  strength  a  lens  of  opposite  character  is  taken  from  the 


$8         REFRACTION  AND  HOW  TO  REFRACT. 

trial-case  and  held  in  apposition  to  it,  and  the  two  lenses 
.are  moved  in  front  of  the  eye  as  a  distant  object  is 
/  observed.  That  lens  or  combination  of  lenses  which  stops 
all  apparent  movement  of  the  object  is  the  correct  neu- 
tralizing lens.  Spherocylindric  lenses  are  neutralized  by 
finding  out  what  sphere  will  correct  one  meridian  and  what 
sphere  will  correct  or  neutralize  the  opposite  meridian  ;  for 
example,  if  a  minus  2  S.  stops  all  movement  in  one 
meridian  and  minus  3  S.  stops  all  movement  in  the  other 
meridian,  then  the  lens  being  neutralized  will  be  plus  2  S. 
combined  with  a  plus  I  cylinder.  Or  after  a  sphere  neu- 
tralizes one  meridian,  a  cylinder  may  be  combined  until  the 
other  meridian  is  neutralized. 


CHAPTER  II. 

THE  EYE.  — THE  STANDARD  EYE.  — THE  CARDINAL 
POINTS.— VISUAL  ANGLE.— MINIMUM  VISUAL  AN- 
GLE.—STANDARD  ACUTENESS  OF  VISION.— SIZE  OF 
RETINAL  IMAGE.— ACCOMMODATION.— MECHANISM 
OF  ACCOMMODATION.— FAR  AND  NEAR  POINTS.— 
DETERMINATION  OF  DISTANT  VISION  AND  NEAR 
POINT.— AMPLITUDE  OF  ACCOMMODATION.  — CON- 
VERGENCE.—ANGLE  GAMMA.— ANGLE  ALPHA. 

The  Eye. — While  the  eye  is  considered  as  the  organ  of 
vision,  vet  its  function  is  to  form  upon  its  retina  an  inverted 
image  of  any  object  looked  at ;  and  if  the  retinal  image  is 
distinct,  the  object  will  appear  distinct ;  if  the  retinal  image 
is  blurred,  the  object  will  appear  blurred.  By  means  of 
the  optic  nerve  and  tract  the  retinal  impression  or  image 
is  placed  in  communication  with  the  brain,  which  interprets 
the  image  and  completes  the  visual  act. 

The  Standard  Eye. — For  purposes  of  exact  calcula- 
tions it  has  been  found  necessary  to  project  a  standard  or 
schematic  eye,  whose  nodal  point  (optic  center)  shall  be 
^seven  millimeters  back  of  the  anterior  surface  of  the  cor- 
-  11  ea  and  fifteen  millimeters  from  the  fovea  (Helmholtz). 
Allowing  one  millimeter  for  the  thickness  of  the  choroid 
and  sclera,  such  an  eye  would  have  an  anteroposterior 
•measurement  of  about  twenty-three  millimeters.  Parallel 
rays  of  light  passing  into  such  an  eye  in  a  state  of  rest 
would  focus  on  the  macula. 

Cardinal  Points  (Fig.  61). — Images  formed  upon  the 
retina  are  the  result  of  refraction  by  three  refracting  sur- 
faces and  three  refracting  media.  The  refracting  surfaces 

59 


• 

6O         REFRACTION  AND  HOW  TO  REFRACT. 

are  the  anterior  surface  of  the  cornea  and  the  anterior  and 
posterior  surfaces  of  the  crystalline  lens.  The  refracting 
media  are  the  cornea  (and  aqueous  humor  forming  a  convex 
lens),  the  crystalline  lens,  and  the  vitreous  humor.  These 
refracting  surfaces  and  media  represent  a  compound  dioptric 
system,  centered  upon  /the  optic  or  principal  axis — i.  e.,  a 
line  drawn  from  the  pole  of  the  cornea  to  a  point  between 
the  nerve  and  fovea.J 

^On  the  principal  axis  are  situated  the  anterior  and  poste- 
rior principal  foci  (see  p.  33),  the  anterior  and  posterior  nodal 


FIG.  61. 

points,  and  the  anterior  and  posterior  principal  points.  The 
anterior  principal  focus  is  situated  upon  the  optic  axis 
N  / 13.745-)- mm.  in  front  of  the  corneal  apex.  The  pos- 
terior principal  focus  is  situated  15.61+ mm.  back  of  the 
posterior  surface  of  the  lens\  (The  nodal  points  are  situ- 
ated about  7  mm.  back  of  the  cornea,  and  correspond 
approximately  to  the  optic  center  of  this  compound  re- 
fracting system  ;  and  as  they  are  so  close  together,  they  are 
considered  as  one  for  all  purposej)  in  the  study  of  the  for- 
mation of  images.  The  first  or  anterior  principal  point  is 


MINIMUM    VISUAL   ANGLE.  6 1 

situated  1.75  mm.  back  of  the  anterior  corneal  surface,  and 
the  second  or  posterior  principal  point  is  situated  2.10  mm. 
behind  the  anterior  surface  of  the  cornea.  The  principal 
points  are  so  closely  situated  that  they  are  considered  as 
one.  The  anterior  focal  distance  equals  15.49+  mm.  and 
the  posterior  focal  distance  equals  20.71  +  mm. 

The  Visual  Angle,  or  Angle  of  View. — The  visual  angle 
is  the  angle  formed  by  rays  of  light  from  the  extremes  of  an 
object  passing  to  the  nodal  point  of  the  eye ;  (or  the  visual 
angle  may  be  defined  as  the  angle  which  the  object  subtends 
at  the  nodal  point) of  the  compound  refracting  system  of 
the  eye.  Rays  of  light  from  the  extremes  of  an  object 


FIG.  62. 

directed  to  the  nodal  point  of  the  eye  pass  through  unre- 
fracted,  and  continuing  their  straight  course,  fall  upon  the 
retina,  forming  an  inverted  image  of  the  object.  (See  Fig. 
62.) 

The  size  of  the  retinal  image  depends  upon  the  size  and 
the  distance  of  the  object  from  the  nodal  point  of  the  eye. 
/  Objects,  therefore,  which  are  seen  under  the  same  visual 
angle  must  have  the  same  sized  retinal  image.  (See  Fig. 
63.) 

If  the  arrows  i,  2,  3,  and  4  represented  a  child,  a  man,  a 
tree,  and  a  church,  respectively  (some  distance  apart),  they 
would  form  the  same  sized  retinal  images,  and  if  the  eye 
\\cre  guided  alone  by  the  size  of  the  retinal  image,  it  would 


62 


REFRACTION  AND  HOW  TO  REFRACT. 


judge  erroneously;  but,  by  experience,  distance  and  com- 
parison of  size  are  brought  into  judgment. 

If,  however,  arrows  2,  3,  and  4  are  placed  at  the  side  of 
arrow  I,  then  their  resulting  images  would  increase  in  size 
according  to  the  size  of  their  respective  visual  angles.  (See 
Fig.  64.) 

The  nearer  an  object  to  the  eye,  the  larger  the  visual 


FIG.  63. 

angle  and  retinal  image  ;  the  further  away  an  object  from  the 
eye,  the  smaller  the  visual  angle  and  retinal  image.  An  ob- 
ject, to  retain  the  same  sized  visual  angle,  must,  therefore,  be 
made  larger  the  further  it  is  removed  from  the  eye  ;  this  is 
demonstrated  in  figure  63,  where  arrow  I,  to  be  seen 


FIG.  64. 

under  the  same  visual  angle  which  it  has  at  present,  would 
have  to  be  as  large  as  arrow  4,  at  the  distance  of  arrow  4. 
Minimum  Visual  Angle. — This  is  the  smallest  visual 
angle  in  which  a  standard  eye  can  still  recognize  an  object 
and  give  it  a  name  ;  this  angle  is  also  spoken  of  as  the 


SIZE    OF    RETINAL    IMAGES.  63 

limiting  angle  of  vision.  In  figure  65,  for  example,  the 
letter  D  at  a  distance  of  six  meters  is  recognized  as  the 
letter  D  :  it  is  plainly  seen  ;  but  if  placed  beyond  six  meters, 
it  would  form  a .  smaller  visual  angle,  and  could  not  with 
certainty  be  called  D. 

To  be  seen  at  a  distance  of  twelve  meters  and  still 
occupy  this  same  visual  angle,  D  would  have  to  be  made 
twice  as  large — /.  e.,  the  size  of  F ;  and  to  be  seen  at 
twenty-four  meters,  it  would  have  to  be  four  times  its  pres- 
ent size,  or  the  size  of  P.  Thus,  while  the  letter  D,  seen 
clearly  at  six  meters,  would  have  to  be  made  proportion- 
ately larger  as  it  is  'removed  from  the  eye,  then  to  occupy 


FIG.  65. 

the  same  visual  angle  it  would  have  to  be  made  smaller 
if  brought  closer  to  the  eye  and  kept  within  this  limiting 
angle.  In  figure  65  D,  F,  and  P  can  be  seen  closer  to 
the  eye  than  their  respective  distances  call  for  ;  but  the  pur- 
pose is  to  find  the  greatest  distance  from  the  eye  at  which 
they  can  be  seen,  as  this  represents  the  maximum  acuteness 
of  vision,  or  maximum  sharpness  of  sight. 

Standard  Acuteness  of  Vision. — As  it  was  necessary  for 
purposes  of  calculation  to  have  a  standard  or  emmetropic 
eye,  so  it  is  essential  to  have  a  standard  acuteness  of  vision 
which  will  be  consistent  with  the  standard  or  emmetropic 
eye,  and  thus  have  some  method  of  recording  numerically 
any  departure  from  this  standard  visual  condition. 


64         REFRACTION  'AND  HOW  TO  REFRACT. 

The  standard  acuteness  of  vision  is  the  power  of  the  eye  to 

x  distinguish  letters  and  characters  occupying  an  angle  of  five 

minutes.    (Every  letter  is,  therefore,  so  proportioned  that  it 

will    measure  just   five  minutes  in  the  vertical  and  hori- 

zontal meridians,  and  be  reducible  to  twenty-five  parts  or 

squares,  each  measuring 
one  minute  vertically  and 
horizontally.*  (See  Fig. 
66.)) 
* 


FIG.  66.  FIG.  67. 

letter  F  drawn  in  a  five- 

minute  square,  and  each  stroke  of  the  letter,  and  space 
between  the  strokes,  measuring  just  one  minute  in  width. 
As  the  tangent  of  the  angle  of  five  minutes  is  expressed 
by  the  decimal  .001454,  then  to  calculate  the  size  of 
any  letter  or  character  which  should  be  seen  clearly  and  dis- 
tinctly by  the  standard  eye  at  a  certain  definite  distance,  it 
is  necessary  to  multiply  the  distance  in  millimeters  by  this 
tangent  of  the  angle  of  five  minutes.  Letters  or  characters 
made  on  this  scale  are  called  standard  letters.  For  ex- 
ample, letters  to  be  seen  under  an  angle  of  five  minutes  at 
a  distance  of  one  meter  (1000  mm.)  would  have  to  be 
1.454  mm.  square  (1000  X  .001454).  At  six  meters 
(6000  X  .001454)  =  8.7  mm^  etc. 

Size  of  Retinal  Images.-^-The  size  of  the  retinal  image 
depends  upon  two  factors  —  the  size  of  the  object  itself  and 
its  distance  from  the  nodal  point)  In  the  standard  eye  it 
has  been  stated  that  the  nodal  point  was  7  mm.  back  of 
the  cornea  and  1  5  mm.  in  front  of  the  retina  ;  then  an 
object  8.7mm.  square  situated  6000  mm.  in  front  of  the 

*  There  are  two  letters  in  the  alphabet  which  are  exceptions  to  this  rule, 
L  and  O.  L  can  be  seen  under  an  angle  of  two  minutes  and  O  can  be  seen 
under  an  angle  of  three  minutes. 


SIZE    OF     RETINAL     IMAGES. 


/£ye  would  have  a  retinal  image  -g-^f^  of  8.7,  or  0.02  -f-  mm., 
and  this  is  the  size  of  the  retinal  image  in  a  standard  eye, 
^looking  at  a  standard  letter  at  six  meters'  distance.  A  good  A.^ 
rule  for  finding  the  size  of  the  retinal  image  is  to  multiply 
the  height  of  the  object  by  the  nodal  distance  and  divide 
by  the  distance.  /In  other  words,  the  size  of  the  retinal 
image  is  to  the  size  of  the  object  as  their  respective  dis- 
tances from  the  nodal  point. 

Refraction  in  ophthalmology  has  most  to  do  with  eyes 
whose  measurements  are  not  according  to  the  standard  or 
emmetropic  condition,  and  which  have  their  retinas  closer 
to  or  further  from  the  nodal  point  than  15  mm.  (spoken  of 
as  ametropic).  The 

retinal     images     in  /^~    ~~~~N£  M 

such  eyes  will  be 
smaller  in  the  former 
and  larger  in  the 
latter.  (See  Fig.  68.) 

Accommodation.  FIG.  6s. 

/ — This  may  be  de- 

>j  scribed  as  the  power  of  the  eye  to  focus  rays  of  light  upon  its 
retina  from  different  distances  at  different  times.  In  other 
words,  the  eye  can  not  focus  rays  of  light  upon  its  retina  from 
different  points  at  one  and  the  same  time.  For  example,  the 
point  of  a  pencil  held  six  inches  in  front  of  the  eye  is  not  seen 
clearly  (is  hazy)  as  the  eye  looks  at  a  printed  page  thirteen 
inches  beyond  ;  and,  vice  versa,  the  printed  page  is  not  seen 
distinctly  if  the  point  of  the  pencil  is  looked  at.  In  the 
study  of  convex  lenses  it  was  noticed  that  when  an  object 
was  brought  closer  than  infinity,  the  focus  of  the  lens  was 
correspondingly  lengthened  ;  and  so,  in  the  photographer's 
camera,  to  keep  the  focus  on  the  ground-glass  or  sensitive 
plate  as  the  object  is  brought  toward  the  camera,  it  is 
6 


66         REFRACTION  AND  HOW  TO  REFRACT. 

necessary  to  push  the  lens  forward  by  means  of  the  accor- 
dion plaits ;  but  the  human  eye  does  not  lengthen  or 
shorten  in  this  way.  Normally,  the  eyeball  is  inextensible, 
and  to  accomplish  this  same  purpose  the  ciliary  muscle 
must  contract,  causing  the  crystalline  lens  to  become  more 
convex,  and  thus  keep  the  rays  of  light  entering  the  eye  at 
a  focus  upon  the  fovea. 

The  Mechanism  of  Accommodation. — To  appreciate 
this,  it  is  necessary  to  understand  something  of  the  anatomy 
of  the  ciliary  body,  of  which  the  ciliary  muscle  is  a  part. 
The  ciliary  body  is  circular  in  form  and  occupies  a  small 
.  (3  mm.)  area  in  the  eye,  just  beneath  the  sclera,  at  its  cor- 
neal  junction.  (See  Fig.  61.)  In  section  the  ciliary  body  is 
triangular  in  shape,  the  base  of  the  triangle  measuring  about 
0.8  mm.  and  facing  toward  the  anterior  chamber,  the  apex 
of  the  triangle  extending  backward  beneath  the  sclerotic. 
The  ciliary  body  lies  in  apposition  to  the  sclera,  but  has 
only  a  very  minute  attachment  to  it,  at  the  sclerocorneal 
junction,  called  the  ligamentum  annulare,  or  pectinatum. 
That  portion  of  the  ciliary  body  lying  next  to  the  hyaloid 
membrane  of  the  vitreous  humor  is  composed  of  folds, 
known  as  the  ciliary  processes,  seventy  or  more  in  number. 

A  portion  of  the  ciliary  body  is  composed  of  muscular 
fibers  disposed  in  flat  bundles,  which  interlace  with  each 
other,  forming  a  sort  of  plexus,  and  called  the  ciliary  mus- 
cle. This  muscle,  by  the  character  of  its  fibers,  has  been 
subdivided  into  three  parts  :  (i)  Meridional  •  (2)  radiating  ; 
and  (3)  circular  or  sphincter  fibers.  The  (meridional  are 
the  longest,  lie  next  to  the  sclerotic  in  lamellae,  parallel 
with  it,  and  pass  back  to  join  the  choroid  coat  of  the  eye,) 
(forming  what  is  known  as  the  tensor  choroidea?,  or  muscle 
|of  Brucke  or  Bowman.  The  radiating  fibers  are  fan-shaped, 
few  in  number,  and  scattered  through  the  ciliary  body. 


THE     MECHANISM     OF    ACCOMMODATION.  6/ 

The  circular  or  sphincter  fibers — also  called  annular — are 
sometimes  referred  to  as  the  muscle  of  Miiller,  or  com- 
pressor lentis,  and  are  the  most  important  fibers  in  the 
consideration  of  accommodation  ;  they  form  a  sphincter 
ring  concentric  with  the  equator  of  the  lens.  Attached  to 
the  ciliary  body,  well  forward  on  its  inner  side,  near  the 
'  base  of  the  triangle,  is  the  ligament  of  the  lens  (zonule  of 
Zinn),  and  it  in  turn  sends  fibers  to  the  anterior  and  poste- 
rior capsule  of  the  lens.  This  ligament  of  the  lens  occu- 
pies an  interval  of  about  0.5  mm.  between  the  ciliary  body 
and  the  periphery  of  the  lens,  and  is  a  constant  factor  in 
all  conditions  of  the  healthy  eye. 

During  the  act  of  accommodation  the  following  changes 
take  place  in  the  eye  : 

1.  The  ciliary^  muscle  contracts. 

2.  The  ciliary  muscle  (sphincter),  by  contracting,  makes 
a  smaller  circle. 

3.  The  tensor  choroideae  draws  slightly  upon  the  choroid 
(compressing  somewhat  the  vitreous  body),  and  these  two 
sets  of  fibers,   sphincter  and   meridional,   acting  together, 

i  relax  the  ligament  of  the  lens,  with  the  result  that — 

4.  The  lens  fibers,  no  longer  held  in  check,  become  re- 
laxed, and  by  their   own  inherent  quality  (elasticity)  allow 
the  lens  to  become  more  convex,  especially  on  its  anterior 
surface. 

5.  The  anterior  surface  of  the  lens  being  made  more 
convex,  approaches  the  cornea. 

|     6.  The  posterior  surface  of  the  lens  becomes  slightly 
/more  convex,  but  retains  its  position  at  the  pole. 

7.  The  lens  axis  is  lengthened,  but  the  equatorial  diame- 
ter diminishes,  thus  keeping  up  the  uniform  interval  between 
the  equator  of  the  lens  and  the  ciliary  body,  as  previously 
referred  to.  The  lens  docs  not  increase  in  volume. 


68 


REFRACTION  AND  HOW  TO  REFRACT. 


8.  The   anterior   chamber  becomes  slightly  shallower  at 
the  center  and  deeper  in  the  periphery. 

9.  That  portion  of  the  iris  resting  upon  the  anterior  cap- 

sule of  the  lens  is  pushed  forward,  espe- 
cially at  its  pupillary  edge. 

10.  The  iris  contracts,  producing  a 
smaller  pupil  ;  but  it  must  be  remembered 
that  contraction  of  the  iris  is  not  an  essen- 
tial condition  in  accommodation.  The  shape 
of  the  cornea  is  not  changed  during  con- 
traction of  the  ciliary  muscle. 

The  following  table  shows  the  compara- 
tive measurements  of  a  lens  at  rest  and 
during  the  height  of  accommodation  in  a 
healthy  emmetropic  eye  of  ten  years.  The  dotted  lines 
in  figure  69  indicate  the  changes  in  the  shape  of  the  lens 
at  the  height  of  accommodation. 


FIG.  69. 


AT  REST. 

Radius  of  curvature  of  anterior  surface  of  lens,  .  10  mm. 

"                 "              posterior       "            "      .  6     " 
Distance  from  anterior  surface  of  cornea  to  ante- 
rior surface  of  lens, 3.6  " 

Anteroposterior  diameter,  on  axis, 3.6  " 

Distance  from  anterior  surface  of  cornea  to  poste- 
rior surface  of  lens, 7-2  " 

Equatorial  diameter, 8.7  " 


HEIGHT  OF 

ACCOMMODATION. 

6      mm. 

5-5   " 


3-2 
4 

7.2 

8.2 


Far  Point. — Latin,  punctum  rcmotum  ;  abbreviated  p.  r. 
^or  r.  The  far  point  may  be  (defined  as  the  greatest  distance 
at  which  an  eye  has  maximum  sharpness  of  sight,  or  the 
most  remote  point  at  which  the  eye,  in  a  state  of  rest,  has 
maximum  acuity  of  vision^)  Infinity  (sign  of  infinity,  oo  )^ 
is  the  far  point  of  an  emmetropic  eye. 

The  standard  or  emmetropic  eye,  when  looking  at  distant' 
objects,  receives  parallel  rays  of  light  at  a  focus   upon  its 


AMPLITUDE    OF    ACCOMMODATION.  69 

fovea  (Fig.  70),  and  also  emits  parallel  rays  ;  under  these 
conditions  the  ciliary  muscle  is  not  acting,  the  eye  is  in  a 
condition  of  complete  repose,  of  rest,  of  minimum  refrac- 
tion, and  is  adapted  for  its  far  point. 

Near  Point. — Latin,  punctual  proximum ;  abbreviated 
p.  p.  or  p.  This  may  be  .defined  as  the  nearest  point  at 
which  an  eye  has  maximum  sharpness  of  sight^  or  the 
nearest  point  to  the  eye  at  which  it  has  distinct  vision,  the 
lens  is  in  the  condition  of  greatest  convexity,  of  maximum 
refraction. 

Amplitude  of  Accommodation. — This  is  also  called  the 
range  *  or  power  f  of  accommodation,  and  may  be^e- 
fined  as  the  difference  between  the  refraction  of  the  eye  in 
a  state  of  rest  (or  adapted  for  its  far  point)  and  in  a  condi- 
tion of  maximum  refraction,  or  adapted  for  its  near  pointy 
For   example,  an  emmetropic  eye  has  infinity  for  its  far 
point,  and  if  10  cm.  distance  is  its  near  point,  then  the  dif- 
ference between  the  lens  adapted  for  infinity  and  locm.  will 
be  10  D.,  as  10  cm.  represents  the  focal  length  of  10  D.     In 
other  words,  there  is  no  accom- 
modation  used  for  infinity,  but 
there  is  an   accommodation  of 
10  D.  for  the  near  point,  which 
is   the   amplitude   or  power  of 
accommodation.      The    emme- 
tropic eye  in  a  state  of  accom-  FIG.  70. 
modation  adds  on  to  the  ante- 
rior  surface    of  its    lens    what   is   equivalent   to    a   convex 
meniscus.      Figure   70  shows   an   emmetropic   eye  at   rest 
receiving   parallel  rays  of  light  at  a  focus   upon   its    retina, 

*  Range  applies  to  the  space  between  the  far  and  near  points, 
f  Power  applies  to  the  force  or  strength  or  diopters  necessary  to  change 
the  refraction  from  the  far  to  the  near  point 


7O  REFRACTION     AM)     HOW 

and  it  also  shows  the  same  eye  in  its  maximum  state  of 
accommodation  for  a  point  10  cm.  distant;  the  broken 
line  representing  what  is  equivalent  to  a  convex  meniscus, 
added  to  the  anterior  surface  of  its  lens. 

When  the  distance  of  the  near  point  is  known  in  inches 
or  centimeters,  the  equivalent  in  diopters  is  found  by  divid- 
ing 40  by  the  near  point  in  inches,  or  by  dividing  roo  by 
the  near  point  in  centimeters.  The  near  point  being  10  cm., 
!  or  4  inches  (10  into  100  or  4  into  40)  the  amount  of  accom- 
modation will  be  10  D. 

In  the  study  of  healthy  emmetropic  eyes  it  has  been 
found  that  the  power  of  accommodation  gradually  dimin- 
ishes as  the  eye  passes  from  youth  to  old  age.  This  is 
the  result  of  one  or  more  changes :  the  lens  fibers  lose 
their  elasticity,  becoming  sclerosed,  or  the  ciliary  muscle 
grows  weak,  or  both  of  these  changes  may  exist  together. 
Rarely  the  cornea  may  flatten.  A  knowledge  of  the  power 
of  accommodation~~isabsolutely  essential,  so  that  any  vari- 
ations from  the  standard  condition  may  be  noted.  The  fol- 
lowing table  gives  the  ages  from  ten  to  seventy-five  years, 
respectively,  with  five-year  intervals,  and  the  near  point 
consistent  with  each,  as  also  the  amplitude  of  accommoda- 
tion for  each  period. 


I  AMPLITUDE  AMPLITUDE 

YEAR.    /NEAR  POINT.     IN  DIOPTERS.       YEAR.     NEAR  POINT.    IN  DIOPTERS. 

10  ></7     cm.  14  45  28cm. /|         3.5 

15  2^8.5    "  12  >  50  40  "  /  ^         2.5 

20  4io      "  10  55  55  ">/  3<?  1.75 

25  -f%i2     ."  8.5  60  loo  "U^/i     * 

30  1457*^               7  ''5  U3  "$'>/>  0.75 

35  18  Jf>                 S-S  7°  400  "              0.25 

40  My'TV             4'5  75  °° 

I  »>  '  /    This  table  of  near  points  applies  only  to  emmetropic  eyes 
/or  those  eyes  which  are  made  emmetropic  by  the  adjust- 


-C.      ^ 

/    '  ^      ,• 


FAR     POINT.  7 1 

ment  of  suitable  correcting  lenses.  The  table  of  amplitudes, 
however,  is  the  same,  with  a  few  exceptions,  for  all  eyes  of 
whatever  degree  or  amount  of  ametropia. 

For  a  better  appreciation  of  the  amplitude  of  accom- 
modation it  is  necessary  to  understand  the  two  forms  of 
eyes  already  referred  to  in  figure  68. 

First,  the  eye  which  has  its  retina  closer  to  its  refractive 
media  than  the  principal  focus  ;  such  an  eye  is  spoken  of 
/as  a  short  or  hyperopic  eye.  (H  in  Fig.  68.)  (Hyperopia  : 
^/  \  Greek,  o-^p,  over;  and  &<!',  eye.) 

This  eye  in  a  state  of  rest  (under  the  influence  of  atro- 
pin)  will  emit  divergent  rays  of  light,  and  is,  therefore,  in  a 


FIG.  71. 

condition  to  receive  only  convergent  rays  of  light  at  focus 
upon  its  retina.  (See  Fig.  71.)  Parallel  rays  would  not 
focus  upon  the  retina  of  such  an  eye,  but,  if  possible,  would 
focus  back  of  the  retina. 

Second,  the  eye  that  has  its  retina  beyond  the  principal 
focus  of  its  dioptric  media  (M  in  Fig.  68) ;  such  an  eye  is 
spoken  of  as  a  long  or  myopic  eye  (Greek,  /wstv,  to  close  ;  «V', 
eye).  This  eye  always  emits  convergent  rays,  and  is,  there- 
fore, in  a  state  to  receive  divergent  rays  of  light  at  a  focus 
upon  its  retina.  (See  Fig.  72.)  Parallel  rays  would  not 
focus  upon  the  retina  of  a  myopic  eye,  but  in  the  vitreous 
in  front  of  the  retina. 

The  Far  Point  of  a  Hyperopic  Eye. — This  must  neces- 


72          REFRACTION  AND  HOW  TO  REFRACT. 

sarily  be  negative  (see  Fig.  71),  and  is  found  by  projecting 
the  divergent  emergent  rays  backward  to  the  imaginary 
point  behind  the  retina  from  which  they  appear  to  have 
diverged.  A  hyperopic  eye,  to  receive  parallel  rays  of 
light  at  a  focus  upon  its  retina,  must,  therefore,  accommo- 
date, and  the  amount  of  accommodation  thus  exerted  will 
remove  the  near  point  just  that  much  from  the  eye  as  com- 
pared with  an  emmetropic  eye.  For  example,  according  to 
the  table  of  amplitudes  just  given,  an  eye  at  twenty  years  has 
I IO  D.  of  accommodation,  but  if  it  uses  2  D.  of  this  to  make 
'  rays  of  light  parallel,  then  it  only  has  8  D.  left  to  accom- 
modate inside  of  infinity,  with  the  result  that  the  near  point 
comes  to  only  (8  into  100)  12.5  cm.  ;  or  an  eye  which  is 


FIG.  72. 

twenty-five  years  old  has  an  amplitude  of  accommodation  of 
8.$  D.,  and  if  it  has  to  use  4.5  D.  for  infinity,  it  would  have 
(4  into  100)  a  near  point  of  25  cm.  (10  inches). 

The  Near  Point  of  a  Hyperopic  Eye. — From  the  de- 
scription just  given  it  will  be  seen  at  once  that  the  near 
point  in  hyperopic  eyes  is  always  further  removed  than  in 
the  emmetropic  eye  for  a  corresponding  age,  and  that  the 
..jar  point  depends  upon  the  amount  of  accommodation  t//' 
that  is  left  after  the  eye  has  accommodated  for  infinity .[  ^  y 

The  Far  Point  of  a  Myopic  Eye. — This  is  always  posi- 
tive and  situated  some  place  inside  of  infinity.  It  is  found 
by  uniting  the  convergent  emergent  rays.  (See  Fig.  72.) 


FAR     POINT. 


73 


The  far  point  of  a  myopic  eye  is  the  result  of  its  strong 
refracting  power  or  the  distance  of  its  retina  beyond  the 
principal  focus  of  its  dioptric  media.  The  retina  and  far 
>  point  of  a  myopic  eye  are  conjugate  foci.  (See  Fig.  72.) 
The  myopic  far  point  is  equivalent  to  just  that  much  refrac- 
tion in  excess  of  the  emmetropic  eye.  An  emmetropic  eye 
under  the  influence  of  atropin  would  require  a  -f-  2  S. 
placed  in  front  of  it  to  make  rays  of  light  focus  upon  its 
retina  from  a  distance  of  50  cm.,  and  rays  of  light  from  the 
retina  of  this  eye  with  a  -j-2  S.  in  front  of  it  would  focus 
at  50  cm.  This  eye,  then,  equals  a  myopic  eye  of  2  D. 
This  myopic  eye  would  have  a  far  point  of  50  cm.  Where 
^  /the  rays  of  light  meet  as  they  come  from  a  myopic  eye  in  a 
I  state  of  rest  is  its  far  point. 

The  Near  Point  of  a  Myopic  Eye. — This  is  always 
^>| closer  than  in  the  emmetropic  eye  for  a  corresponding  age, 
and  depends  upon  the  distance  of  its  far  point.  For  exam- 
ple, an  eye  at  twenty-five  years  has  8.5  D.  amplitude  of 
accommodation,  but  if  it  has  a  far  point  of  70  cm.,  then  its 
near  point  will  be  represented  by  8.5  D.  and  70  cm., — /.  c., 

1.5  D.,  which  would  equal  10  D.,  or  a  near  point  of  10  cm. 

.  ^C-^W^ 

The  following  table  gives  the  comparative  near  points  jn  an 

emmetropic  eye,  a  hyperopic  eye  of  2  D.,  and  a  myopic  eye 
of  2  D.  : 


AGE. 

10 

15 

20 

25 

30 

35 

40 

45 

5° 

55 

60 

65 

70 

75 

F.tnnu-tropia,     p.p. 

7 

8.3 

10 

12 

14 

18 

22 

28 

40 

5S 

100 

UT 

400 

at 

2  1  ).  11  vperopia,  " 

8.3 

10 

125 

16 

20 

28.5 

40 

66 

200 

00 

— 

— 

2  D.  Myopia,        " 

6 

7 

»-3 

10 

11 

13 

15-3 

" 

22 

25 

33 

36 

44 

50 

Determining  the  Vision. — This  may  be  considered  as 

the  method  of  finding  out  what  an  eye  can  see  without  any 

lenses  placed  in  front  of  it ;  in  other  words,  determining  the 

vision  may  be  defined  as  ascertaining  the  seeing  quality  of  the 

7 


74 


REFRACTION  AND  HOW  TO  REFRACT. 


_ 
• 

L   6   A 

T  G"Y  D 

» 
FAVHU 

DLOECN 

CPAR'CEOU 


unrefracted  eye.  The  refraction  of  an  eye  should  never  be 
confounded  with  the  visual  quality,  as  refraction  applies  to 
the  refractive  media  ;  for  example,  an  emmetropic  eye  with 
a  hemorrhage  at  the  fovea  would  be  practically  without 
visual  quality,  and  yet  its  refraction  or  refractive  condition 
would  be  standard.  The  most  acute  vision  is  at  the  fovea 
and  the  region  immediately  surrounding  it,  but  this  sensi- 
bility diminishes  as  the  fovea  is  departed  from  and  the  per- 
ipheral portion  of  the  retina  approached  ;  this  is  due  to  the 

fact  that  the  cones  are  as 
close  as  0.002  mm.  at  the 
macula,  and  not  so  close  or 

C_          f>^  numerous  in  the  forepart  of 

•  ^^         *^        the  eye-ground. 

Test-type  or  Test-let- 

terS  f°r  Distant  vision-- 

To  determine  the  vision  we 

employ  cards  on  which  are 
engraved  test-type  or  letters 
of  various  sizes,  constructed 
so  that  each  letter  subtends 
an  angle  of  five  minutes,  as 
suggested  by  Snellen,  and 
described  on  page  64. 

Figure  73  shows  such  cards  of  test-letters,  reduced  in  size. 
The  Roman  characters  just  over  the  top  of  the  letters  indi- 
cate the  distance  in  meters  that  the  letters  should  be  seen  by 
the  standard  eye,  and  the  little  figures  at  the  left  of  the  letters 
indicate  the  equivalent  distance  in  English  feet.  The  top 
letter  should  be  seen  at  60  meters,  and  the  bottom  letters 
at  3  meters  ;  the  intervening  letters  are  to  be  seen  at  the 
respective  distances  indicated.  As  it  is  not  unusual  to  find 
eyes  that  have  a  seeing  quality  better  than  that  obtained 


O   A   L 

G  YDD  T 

n 
VFHUA 

LE  C~H  D  O 

GEORLCPA 


FIG.  73.—  Randall's  Test-letters.  Block 
letters   in    black    on    cream-colored 

cards. 


TEST-TYPE   OR   TEST-LETTERS   FOR    DISTANT   VISION.        75 


with  Snellen's  type  constructed  on  the  angle  of  five  min- 
utes, Dr.  James  Wallace  has  constructed  letters  which  sub- 
tend an  angle  of  only  four  minutes.  Such  a  card  is  shown 
in  figure  74  and  has  a  large  field  of  usefulness.  While 
test-cards  are  usually  white  or  cream-colored,  with  black 
letters,  Gould  has  white  letters  constructed  on  black 

cards.      (See  Fig.  77.)     As  white  stimu- 

.  Cj  lates  the  retina  and  black   does  not,  it 

*  •  will  be  recognized  at  once  that  in  one 

-Q     ^3  instance  the  card,  and  in  the  other  the 

letters,   produce  the  retinal  stimulation. 

The  white  letters  seem  to  stand  out  from 

the  black  card  al- 
most as  if  they  were 

embossed,    giving    a 

clear-cut    edge    and 

most  soothing  effect 

to  the  eye  under  ex- 
amination, and  can 

be   recognized  when 

subtending    a    much 

smaller    angle    than 

the  black  letters.    To 

this    card  should   be 


-o  o  o 

-X  X  Y  V 

•a  a  8  H  M 

-1  1  O  O  A  R 

•  V  V  T  3  S   X   » 

•*AOO«8JT 


FIG.  74. — Four  Min- 
ute Letters  of  Dr. 
J.  Wallace.  This 
card  is  constructed 
principally  for  re- 
flection purposes. 
(See  Fig.  7.) 


->javoid   reflection, 
hung  at  an  angle. 

For  aliens  who  do  not  know  the 
English  letters,  and  for  illiterates,  a 
special  card  has  been  made,  known  as 
the  illiterate  or  "dummy"  card,  with 
characters  consisting  of  lines  shaped 


111 

3  LU 

E  111  3 

E  3  ui  m 


ui  3  E 


FIG.  75. 


like  the  capital  letter  "  E,"  and  made  to  conform  to  the  five- 
minute  angle.  As  these  letters  are  variously  placed,  the 
patient  is  asked  to  tell,  or  indicate  with  his  finger  or  fingers, 


76 


REFRACTION  AND  HOW  TO  REFRACT. 


the  direction  in  which  the  prongs  of  the  "E"  point:  up, 
down,  to  the  right  or  left.  This  illiterate  card  (see  Fig. 
75)  is  much  to  be  preferred  to  the  German,  Hebrew,  and 
"  figure  "  cards  occasionally  displayed  in  clinics. 

Kindergarten  Card. — To  keep  the  attention  of  children 
who  do  not  know  the  alphabet,  or  who  attend  a  Kinder- 
garten school,  the  author  has 
prepared  a  card  of  test  objects 
as  shown  in  figure  76.  These 
objects  have  been  drawn  up 
to  the  angle  of  five  minutes, 

j  „.  though    some  of  their    com- 

ponent parts  do  not  measure 
Um  up  to  one  minute. 

^^          —  Selection   of  Test-cards. 

^J        f»        iMt         — ^e  surgeon   should   have 

several  of  these  in  duplicate 
with  the  order  of  the  letters 
changed  (Figs.  73,  77),  as  pa- 
tients not  infrequently  and 
unintentionally  commit  them 
to  memory.  Care  should  be 
exercised  in  the  selection  of 
test-cards,  to  see  that  each 
letter  on  the  card  measures 


O 

t 

o  a 


* 

A 


•  a  a  T  •  o  & 


FIG.  76. — Author's  Test  Objects  or 
Kindergarten  Card  for  Children 
and  Illiterates. 


up  to  the  standard  square  of 
five  minutes,  as  many  of  the 
A's  and  R's  and  N's,  etc.,  on 
the  old  cards  as  seen  in  the 

shops  measure  six  and  seven  minutes  horizontally.  It  is 
a  matter  of  choice  with  the  surgeon  whether  to  use  test- 
cards  with  the  block  or  Gothic  letters.  It  is  well  to  have 
both. 

£ 


THE    RECORD    OF    THE    VISUAL    ACUITY. 


77 


Method  of  Procedure. — The  test-card  should  be  hung 
on  the  wall  with  its  ^  line  five  or  six  inches  below  the 
level  of  the  patient's  eyes,,  and  illuminated  by  means  of 
reflected  artificial  light.  This  is  always  a  certain  quantity, 
whereas  daylight  is  too  variable  and  not  to  be  depended 
upon.  The  patient  should  be  placed  with  his  back  toward 
any  bright  light,  and  at  a  distance  of  six  meters  from  the  card. 


FIG.  77. — Gould's  Test-letters.     Gothic  letters  in  white  on  black  cards. 


Sometimes  the  surgeon's  office  is  not  six  meters  long,  and 
this  distance  must  be  obtained  by  using  diagonal  corners 
of  the  room  or  by  using  a  plane  plate-glass  mirror  and  a 
specially  prepared  test-card  with  reversed  letters  (see  Fig. 
74),  the  card  being  hung  as  many  meters  in  front  of  the 
mirror  as  will  make  six  meters  when  added  to  the  length 


78         REFRACTION  AND  HOW  TO  REFRACT. 

of  the  office.      While  a  distance  of  six  meters  is  always  to 
jbe  preferred,  yet  if  this  can  not  be  obtained,  the  surgeon 
I  may  use  a  distance  of  four  meters,  but  never  less  than  this. 
Each  eye  should  be  tested  separately,  the  fellow-eye  being 
shielded  or  covered  by  a  card  or  opaque  disc  held  in  front 
of  it  or  placed  in  the  trial-frame.      The  eye  should  never  be 
d  shut,  and  any  pressure  upon   the  eyeball    must  be 
avoided. 

The  record  of  the  visual  acuity  is  usually  made  in  the 
form    of    fractions,    using    Arabic    or     Roman     notation  ; 
figures  usually  indicate  feet,  and  Roman  letters  usually  sig- 
meters^  though  there  is  no  fixed  rule  for  this.      How- 
ever  expressed,  the   denominator  indicates  the  size  of  the 
I  frjt^typjs  which  the  eye  reads,  at  (The  distance  indicated  by  the 
y  <fr      numerator^    For  example,  if  at    VI  meters  the  eye  reads 
I    ^     jiR6  lme  °f  letters  marked  VI,  then   the  record  would  be 
J^     y|.     This    would    be  |-[J-  if   the  numerator  and  denomina- 
tor were  expressed  in  feet.     If  the  eye,  at  a  distance  of 
VI  meters,  reads   only  the  letters  on  the   XII   line,  then 
the   record  would   be  ^j,  or  |g-  (feet).      If  the  top   letter 
was  the  only  one  recognized  at  the  distance   of  six  meters, 
then  the   record  would  be  ^  (meters),  or  -f^  (feet).     If 
the  eye  reads  the  VI  line,  miscalling  two  letters,  then  the 
record  could  be  made  in  one  of  three  ways,  each  indicating 
the    same    thing.       ~  ?  ?    (one    question    mark     for    each 
miscalled    letter),     or    "y|    partly,"    would     indicate    that 
the  eye  saw  yj,  but  not  each  letter  correctly.     This  way  of 

(making  the  record  is  not  so  explicit  as  that  with  question 
marks.     Or,  ^yj^r+  would  mean  that  the  eye  saw  all  of 


^j^  and  some  of  the  letters  of  -^j- ;  but  this,  too,  is  not  so 
definite  as  the  first  record  and  the  one  recommended. 


DETERMINING    THE    NEAR    POINT.  79 

If  the  eye  can  not  recognize  any  letter  on  the  card  at  the 
distance  of  VI  meters,  then  the  card  should  be  brought 
toward  the  patient,  or  the  patient  told  to  approach  the  card, 
until  the  eye  cznjnst  make  out  the  top  letter  and  no  more. 
If  this  is  seen  at  IV  meters,  then  the  record  will  be  ~  ;  if  at 
one  meter,  the  record  would  be  -  r,  etc.  While  it  has 

l*A 

been  stated  that  the  visual  record  is  usually  made  in  the 
form  of  common  fractions,  as  just- described,  yet  there  are 
some  who  prefer  to  make  the  record  in  the  form  of  deci- 
mals ;  namely,  a  vision  of-—  would  be.i.o,  a  vision  of  - 

7  VI  XII 

would  be  0.50,  or  a  vision   of  j^j^  would  be  0.25,  or  a 

vision  of  --  would  be  o.  I.  "TVIost  authorities  prefer  to  make 
their  records  in  the  form  of  common  fractions. 

In  some  instances  the  eye  may  not  be  able  to  distinguish 
ain-   letter    on  the   card,  no  matter   how  close   it   may  be 
brought  to  the  eye,  and  in  such  a  case  the  vision  is  tested 
holding  the  outstretched  fingers  between  the  patient's 
eye  and  a  bright   light  (an  open  window),  and  a  note  is 
e  of  the  greatest  distance  at  which  the  eye  can  count 
fingers  ^if  at  ten   inches,  the  record   would    be   "  fingers 
counted  at  ten  inches,"  or  whatever  the  distance  may  be. 
/This  ability  to  recognize  form  is  spoken  of  as  "  qualitative 
light   perception."     Eyes  that  are  not  able   to  recognize 
/form  may  still  be  able  to  distinguish  light  from  darkness, 
•C^and  this  ability  is  tested  by  alternately  covering  and  uncov- 
ering the  eye  as  it  faces  a  light,  or  as  light  is  reflected  into 
it  from  a  mirror.     If  "  qualitative  light  perception  "  is  pres- 
ent, the  vision  is  recorded/t^P. ,  which   means  "  light  per- 
jception,"  or  the  record  may  be  made  L.  &  S.,  which  means 
x^lpractically  the  same  thing,  "light  and  shade." 

Determining  the  Near  Point. — Having  obtained  and 
recorded  the  distant  vision  of  an  unrefracted  eye,  it  is  well 


8O          REFRACTION  AND  HOW  TO  REFRACT. 

to  also  find  out  and  note  what  is  the  nearest  point  to  the 
eye  at  which  small  type  can  be  made  out ;  this  is  spoken 
of  as  determining  the  near  point. 

Test-type  or  Test-letters  for  Near  Vision. — To  deter- 
mine the  near  point,  we  employ  cards  on  which  are  printed 
or  engraved  words  or  sentences,  or  a  series  of  letters,  so 
that  each  letter  in  each  word  or  sentence  shall  subtend  an 
angle  of  five  minutes,  at  a  given  distance  from  the  standard 
eye ;  for  instance,  letters  that  are  to  be  seen  at  one  meter 
and  occupy  the  angle  of  five  minutes,  must  be  1.425  mm. 
square ;  letters  that  are  to  be  seen  at  half  a  meter  distance 
must  be  0.712  mm.  square,  etc.  (Most  of  the  "near"  cards 
in  the  market  are  very  defective  in  this  respect,  and  the  near 
types  of  Jaeger  are  becoming  obsolete,  as  they  are  not 
standard  letters,  but  merely  represent  the  various  fonts  of 
printers'  type.)  The  writer's  card  is  one  of  Gothic  type,  as 
shown  in  figure  78.  Another  card  in  block  letters  is  shown 
in  figure  79.  Above  each  series  of  letters  is  marked  the 
greatest  distance  (D)  at  which  the  respective  letters  can  be 
3een;  these  distances  vary  from  0.25  to  2  meters  (25  to 
200  cm.),  which  are  ample  for  all  purposes  in  estimating 
the  near  point. 

Method  of  Procedure  to  Find  the  Near  Point. — The 
patient  is  seated  so  that  the  light  entering  the  room  will 
come  over  his  shoulder  and  fall  upon  the  card  of  test-type 
held  in  front  of  him.  The  surgeon,  to  one  side  of  the 
patient,  holds  the  card  in  one  hand  and  a  meter  stick  in  the 
other,  the  eye  which  is  not  being  tested  is  covered  with  a 
card,  and  the  patient  is  told  to  select  the  smallest  type  on 
the  card  which  he  can  read  or  spell,  and  as  he  continues  to 
do  so  (aloud),  the  surgeon  gradually  approaches  the  card 
to  the  eye  until  the  patient  says  that  the  letters  commence 
to  grow  "hazy"  and  he  can  scarcely  decipher  them;  or 


TEST-TYPE    FOR     NEAR     VISION. 


81 


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REFRACTION     AND     HOW     TO     REFRACT. 


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CONVERGENCE.  83 

.'another  way  is  to  hold  the  card  close  to  the  patient's  eye 
'and  gradually  withdraw  it  until  he  can  just  recognize  the 
:  letters  ;  when  this  point  is  reached,  the  distance  from  the  eye 
to  tlie  card  is  measured  with  the  meter  stick,  and  this  dis- 
tance,  as  also  the  size  of  the  type  which  was  read,  is  care- 
fully  recorded.  For  example,  the  patient  selecting  the  type 
marked  0.50  D.  and  is  able  to  read  it  as  close  as  8  cm.  and 
no  closer,  the  record  will  be  "  near  point  equals  type  0.50 
D.  at  8  cm.";  or  abbreviated,  would  be  "type  0.50  D. 


=  8  cm." 

In  some  instances  the  patient  may  not   be  able  to  read  • 


any  of  the  near   type  without  the  aid  of  a  glass,  and  if  so, 

it  will  be  necessary  to  place  a  plus  sphere  in  front  of  the 

eye  to  assist  in  finding  the  near  point  ;  for   example,  if  a 

+  28.  was   employed,    then   the   record   might   be  "near     /'  .-  -  - 

point  equals  type  0.50  D.  at  12  cm.  with  -}-2  S.,"  or  "  -{-2    /V^ 

S.  =  type  0.50  D.  at  12  cm." 

Convergence.  —  Con,  "  together,"  and  vergere,  "  to 
turn  "  ;  literally,  turning  together.  This  is  the  power  of  the 
internal  recti  muscles  (especially)  to  turn  the  eyes  toward 
the  median  line  ;  to  "  fix"  an  object  closer  than  infinity. 
Standard  eyes,  when  looking  at  an  object  at  a  distance  of 
six  meters  or  more,  are  not  supposed  to  converge  ;  the 
visual  lines  are  spoken  of  as  parallel  and  the  power  of  con- 
vergence is  in  a  state  of  repose.  The  angle  which  the 
visual  line  makes  in  turning  from  infinity  (  oc)  to  a  near 
point  is  called  -th~e"angle  of  convergence,  and  the  angle 
which  is  formed  at  one  meter  distance  by  the  visual  axis 
with  the  median  line  is  called  tlTerneter  angle,  or  the  unit 
of  the  angle  of  convergence.  (See  I,  in  Fig.  80.) 

If  the  visual  line  meets  the  median  plane  at  ^  of  a 
meter,  it  has  then  two-meter  angles  of  convergence  ;  at 
1^  of  a  meter,  four-meter  angles  of  convergence,  etc.  Or 


84 


REFRACTION  AND  HOW  TO  REFRACT. 


five-meter  angles  means  that  the  eye  is  converging  to  a 
point  ^  of  a  meter  distant. 

The  size  of  the  meter  angle  varies  ;  it  is  not  the  same  in 
all  individuals  ;  in  fact,  the  meter  angle  is  smaller  in  children 
than  in  adults,  as  a  rule,  on  account  of  the  shorter  inter- 
ocular  distance.  In  children  thisliistance  is  about  50  mm., 
whereas  in  adults  it  is,  on  the  average,  60  or  64  mm. 

While  standard  eyes,  to  see  a  point  one  meter  distant 
would  converge  just  one  meter  angle,  they  would  also 
accommodate  just  one  diopter  ;  to  see  a  point  at  ^  of  a 


FIG.  80. 

meter  they  would  converge  just  three  meter  angles,  and  at 
the  same  time  would  accommodate  three  diopters,  etc., 
thus  showing  how  intimately  the  powers  of  convergence  and 
accommodation  are  linked  together,  though  it  is  possible 
to  converge  without  accommodation  (see  Presbyopia)  or  to 
accommodate  without  convergence  (paralysis  of  the  in- 
terni). 

Far  and  Near  Points  of  Convergence. — Just  as  we  have 
a  far  and  a  near  point  of  accommodation,  we  also  have  a  far 
and  a  near  point  of  convergence.  The  far  point  of  conver- 
gence is  the  point  to  which  the  visual  lines  are  directed 
when  convergence  is  at  rest,  or  at  a  minimum.  The  near 


ANGLE    GAMMA.  85 

point  of  convergence  is  the  point  to  which  the  visual  lines 
are  directed  when  the  eyes  are  turned  inward  to  their 
utmost  degree. 

Infinity,  or  parallelism,  is  the  position  of  the  visual  lines 
in  the  standard  eyes  in^a  state  of  rest  (E  o>,  in  Fig.  80). 
Visual  lines  that  diverge  in  a  state  of  rest  can  only  meet  by 
being  projected  backward,  and,  therefore,  meet  at  an  imag- 
inary point  behind  the  eyes  (N,  in  Fig.  80) ;  convergence  is 
then  spoken  of  as  negative,  or  minus  ( — ). 

If  the  visual  lines  meet  in  a  state  of  rest,  then  conver- 
gence is  spoken  of  as  positive  (-f ). 

The  amplitude  of  convergence  is  the  distance  measured 
from  the  far  point  to  the  near  point  of  convergence,  and  is 


FIG.  81. 

represented  by  the  greatest  number  of  meter  angles  of  con- 
vergence which  the  eyes  can  exert. 

Angle  Gamma. — An  understanding  of  what  is  known  as 
the  angle  gamma  is  important,  that  the  observer  may 
understand  and  appreciate  the  real  or  apparent  position  of 
the  eyes  when  looking  at  a  near  or  distant  point.  Figure  8 1 
shows  the  line  O  A  (optic  axis)  and  the  optic  center,  or 
nodal  point  (N),  is  situated  on  this  line  in  the  posterior  part 
of  the  crystalline  lens.  The  line  V  M  is  really  a  secondary 
axis  to  this  dioptric  system  of  the  eye,  and  unites  the  object 
(V)  with  the  fovea  centralis  at  M  ;  this  line  is  known  as  the 


tst 


fc^X.        f       m 

t£t*^tte/  ?^&9 

S3    -a.    ->^^*^ 

86  REFRACTION 


•/v 


V-  tf 


AND    HOW    TO    REFRACT. 


"^H&*«  4-^ 

#7w< 

'  ^   '^^        S~  ' 


visual  line.  '  The  angle  formed  by  the  visual  line  with  'the 
axis  at  the  nodal  point  may  be  considered  as  the  angle 
gamma.*  . 

If  the  fovea  centralis  at  M  was  situated  on  the  optic  axis 
»  ~*r~&  A,  then  the  visual   line  and  optic  axis  would  coincide, 
and  there  would  not  be  any  angle  gamma. 

In  hyperopic  and  emmetropic  eyes  the  outer  extremity 

of  the  visual  line  lies  $,  7, 
'or,  in  some  instances,  as 
much  as  10  degrees  to  the 
nasal  side  of  the  optic  axis 
(averaging  about  $  de- 
grees), and  is  spoken  of 
as  positive,  and  given  the 
plus  sign.  In  some  long 
myopic  eyes,  however,  the 
outer  extremity  of  the 
visual  line  may  lie  to  the 
outer  side  of  the  optic 
axis,  when  it  is  spoken  of 
as  negative,  and  given  the 
minus  sign. 

To  demonstrate  the  an- 
gle gamma,  the  patient  is 

told  to  look  at  the  point  of  a  pencil  or  pen  held  in  the 
hand  of  the  surgeon,  about  1 3  inches  distant  (A  in  Figs. 
82  and  83).  If  the  angle  gamma  is  positive,  the  eyes  will 
appear  divergent  to  the  observer,  who  looks  at  the  position 

*  This  is  not  a  perfectly  correct  statement,  as  the  real  angle  gamma  is  the 
angle  formed  by  the  line  of  fixation  V  R  with  the  optic  axis,  R  being  the 
center  of  rotation.  The  angle  V  N  O  and  the  angle  V  R  O  being  so  nearly 
equal,  are,  for  all  intents  and  purposes,  considered  as  the  same. 


FIG.  82. 


ANGLE    ALPHA. 


of  the  poles  of  the  corneas  or  centers  of  the  pupils.      (See 
Fig.  82.) 

If  the  angle  gamma  is  negative  the  eyes  will  appear 
convergent — that  is,  they  appear  to  converge  to  a  point  in 
front  of  the  pencil.  (See 
Fig.  83.) 

The  amount  of  the 
angle  gamma  can  be 
measured  by  using  the 
arc  of  the  perimeter  held 
horizontally,  the  patient 
being  placed  in  the  same 
position  as  when  having1 
his  field  taken.  To  do 
this,  while  the  eye  fixes 
the  central  point,  the 
surgeon  passes  a  candle- 
flame  along  the  arc  until 
the  catoptric  image  of 
the  flame  is  seen  at  the 
center  of  the  pupil ;  this  F  g 

position   of  the   candle- 
flame  on  the  arc  is  noted  in  degrees,  which  is  the  size  of  the 
angle  gamma. 

Angle  Alpha. — This  is  the  angle  formed  by  the  long 
axis  of  the  corneal  ellipse  with  the  visual  axis.  In  the 
consideration  of  this  angle  it  must  be  remembered  that  the 
cornea  resembles,  in  its  central  area,  at  least,  an  ellipsoid 
of  revolution,  with  the  shortest  radius  usually  in  the  verti- 
cal meridian.  The  angle  alpha  is  spoken  of  as  positive 
when  the  outer  extremity  of  the  long  axis  of  the  cornea  is 
to  the  outer  side  of  the  visual  line,  and  negative  when  it 
is  to  the  nasal  side. 


< 


CHAPTER  III. 

OPHTHALMOSCOPE. 

DIRECT  AND  INDIRECT  METHOD. 


Ophthalmoscope.  —  From  oyffa^x;,  "eye,"  and  axo-siv, 
"  to  observe  "  or  "  view  ";  literally,  "  to  view  an  eye."  An 
instrument  used  for  studying  the  media  and  interior  of  the 
eye.  The  pupil  of  an  eye  in  health  appears  to  an  observer  as 
black  ;  this  is  due  to  the  fact  that  the  observer's  eye  does 
not  ordinarilyintercept  any  of  the  rays  of  light  which  return 
from  the  eye.  Rays  of  light  entering  an  eye  are  returned 
toward  their  immediate  source,  and,  therefore,  if  an  observer 
wishes  to  see  into  or  study  the  interior  of  an  eye,  he  must 
have  his  own  eye  in  the  path  of  the  returning  rays.  To 
accomplish  this,  the  observer  places  a  mirror  in  front  of  his 
eye,  so  that  the  reflected  rays  entering  the  eye  are  returned 
toward  the  mirror.  There  is  an  infinite  variety  of  these  in- 
struments in  the  market,  but  for  the  general  student  the 
modified  instrument  of  Loring  appears  to  meet  with  most 
favor.  (See  Fig.  84.) 

This  has  a  concave  mirror  with  a  radius  of  curvature  of 
40  cm.,  giving  a  principal  focus,  therefore,  at  20  cm.  The 
sight-hole  is  round  and  about  3  ^  mm.  in  diameter,  cut 
through  the  glass  ;  this  mirror  can  be  tilted  to  an  angle  of 
25  degrees.  As  an  improvement  over  such  a  mirror,  and  to 
take  its  place,  the  writer  would  recommend  the  mirror  used 
on  his  own  ophthalmoscope,  which  has  a  radius  of  curvature 
of  15  cm.;  and  the  sight-hole,  2^  mm.  in  diameter,  is  not 
cut  through  the  glass,  but  is  made  by  removing  the  quick- 

88 


OPHTHALMOSCOPE. 


89 


silver.  The  glass  at  the  sight-hole  gives  additional  reflect- 
ing surface,  and  at  the  same  time  does  away  with  much 
annoying  aberration  which  results  when  the  glass  is  per- 
forated. 


FRONT 


BACK 


FIG.  84. 


The  small  sight-hole  is  an  advantage,  also,  in  looking 
into  small  pupils.  The  mirror,  oblong  in  shape,  18  by  33 
mm.,  is  secured  at  the  center  of  its  ends,  by  two  elevated 
screws,  to  a  hollow  disc  4^  cm.  in  diameter,  in  which  is  a 
revolving  milled  wheel,  containing  small  spheres,  each 
about  6  mm.  in  diameter.  The  series  of  spheres  ranges 
8 


9O         REFRACTION  AND  HOW  TO  REFRACT. 

from  — I  D.  to  —8  D.,  and  from  -f  I  D.  to  +7  D.     The 
central  aperture  does  not  contain  a  lens,  but  is  left  open. 

When  it  is  desirable  to  use  any  lens  stronger  than  — 8 
D.  or  +  7  D.,  there  is  an  additional  quadrant,  which  can  be 
superimposed  and  turned  into  place  at  the  sight-hole  ;  it 
contains  four  lenses,  — 0.50  D.  and  — 16  D.,  also  +0.50 
D.  and  -{-16  D.  With  this  quadrant  and  the  spheres  in 
the  milled  wheel,  any  spheric  combination  can  be  made 
from  zero  to  — 24  D.  or  to  +  23  D.  An  index  below  the 
sight-hole  of  the  instrument  records  the  strength  of  lens 


FIG.  85. 

that  may  be  in  use  ;  minus  lens.es  are  usually  marked  in  red 
and  plus  lenses  in  white.  pKwU^i  V.\ 

How  to  Use  the  Ophthalmoscope. — There  are  two  ways 
or  methods  by  which  the  ophthalmoscope  may  be  used — 
the  direct  and  the  indirect. 

The  Direct  Method  (see  Fig.  85). — Proficiency  with 
the  ophthalmoscope  does  not  come  except  from  long  and 
constant  practice,  and  several  important  matters  should 
receive  very  careful  attention  before  the  student  attempts  to 
study  the  interior  of  an  eye. 


OPHTHALMOSCOPE.  9! 

The  Room. — This  should  be  darkened  by  drawing  the 
shades  or  closing  the  blinds  ;  the  darker  the  room,  the  better. 

The  Light. — This  should  be  steady,  clear,  and  bright ; 
a  good  lamp  is  suitable,  but  an  Argand  burner  gives  more 
intense  light,  and  is  to  be  preferred,  especially  if  it  is  placed 
on  an  extension  bracket  that  can  be  raised  or  lowered 
and  is  capable  of  lateral  movement. 

Position  of  Light  and  Patient. — The  light  should  be 
several  inches  to  one  side  and  back  of  the  patient,  and  on  a 
level  with  the  patient's  ear,  so  as  to  illuminate  the  outer  half 
of  the  eyelashes  of  the  eye  to  be  examined  ;  it  may  even 
be  well  to  have  the  tip  of  the  patient's  nose  illuminated. 

The  patient  should  be  seated  in  a  comfortable  chair 
(without  arms),  and  is  instructed  to  look  straight  ahead  into 
vacancy,  or  at  a  fixed  object  if  necessary,  and  is  only  to 
change  the  direction  of  his  vision  when  told  to  do  so. 
Under  no  circumstances  should  the  patient  be  allowed  to 
look  at  a  light,  as  this  will  contract  the  pupil. 

For  the  beginner,  it  may  be  well  to  dilate  the  patient's 
pupil  with  a  solution  of  cocain  or  homatropin.  The 
student,  however,  should  learn  as  soon  as  possible  to  see 
into  an  eye  without  the  aid  of  a  mydriatic,  as  many  patients 
seriously  object  to  the  slight  inconvenience  that  results 
from  the  drugs  mentioned. 

The  Observer. — If  the  observer  has  any  decided  refrac- 
tive error,  he  should  wear  his  correcting  glasses  ;  the  reason 
for  this  will  be  explained  later.  The  observer  should  be 
seated  at  the  side  of  the  patient  corresponding  to  the  eye 
he  is  to  examine.  Examining  the  right  eye,  the  observer 
should  be  on  the  patient's  right ;  if  the  left  eye,  then  on 
the  patient's  left. 

"When  examining  the  right  eye,  the  ophthalmoscope  is 
held  in  the  right  hand,  before  the  right  eye ;  and  in  the  left 


92         REFRACTION  AND  HOW  TO  REFRACT. 

hand,  and  before  the  left  eye,  when  examining  the  left  eye. 
The  silrgeon's  eye  should  be  a  little  higher  than  the  patient's. 
and  observer  should  keep  both  eyes  open.     The 
exception  to  this  is  when  the  patient    has  a  squint, 
when  it  will  be  necessary  for  him  to  cover  the  eye  not  being 
examined,  and  in  this  way  the  eye  under  observation  will  look 
straight  ahead. 

The  surgeon  holds  the  ophthalmoscope  perpendicularly, 
so  that  the  sight-hole  in  the  mirror  is  directly  opposite  to 
his  pupil  and  close  to  his  eye.  The  side  of  the  instrument 
rests  on  the  side  of  his  nose  or  the  upper  margin  is  in  the 
hollow  of  the  brow.  The  mirror  is  tilted  toward  the  light, 
surgeon's  elbow  should  be  at  his  side,  and  not  form  an 
angle  with  his  body. 

With  these  several  details  carefully  executed,  the  surgeon" 
^begins  his  examination  at  a  distance  of  about  25  or  30  cm., 
never  closer ;  and  at  this  distance  he  reflects  the  light  from 
the  mirror  into  the  eye,  and  observes  a  "  red  glare,"  which 
occupies  the  previously  black  pupil.  This  is  called  the 
"  reflex,"  and  is  due  to  the  reflection  from  the  choroidal 
coat  of  the  eye.  The  color  of  the  reflex  varies  with  the  size 
of  the  pupil,  transparency  of  the  media,  the  refraction,  and 
the  amount  of  pigment  in  the  eye-ground. 

Having  obtained  the  "  reflex,"  it  will  be  well  for  the  be- 
ginner to  practise  keeping  the  reflected  light  upon  the  pupil 
by  changing  his  distance,  approaching  the  eye  as  close  as 
an  inch  or  two  ;  this  must  be  done  slowly,  and  not  with  a 
rush. 

What  the  Observer  Sees. — Having  learned  to  keep  the 
light  on  the  pupil,  the  next  thing  is  to  study  the  transparency 
of  the  media — i.  e.,  to  find  out  if  there  is  any  interference 
with  the  free  entrance  and  exit  of  the  reflected  rays,  such 
as  would  be  caused  by  opacities  in  the  cornea,  lens,  lens 


OPHTHALMOSCOPE.  93 

capsule,  or  vitreous  ;  and,  if  present,  to  note  their  character 
and  exact  location,  whether  on  the  visual  axis  or  to  one 
side,  etc.  The  next  objective  points  will  be  mentioned 
individually,  and  with  the  idea  of  systematizing  the  study. 

The  Optic  Nerve. — Also  called  the  disc  or  nerve  head 
or  papilla. 

Color  of  the  Optic  Disc. — This  has  been  described  as 
resembling  in  color  the  marrow  of  a  healthy  bone,  or  the 
pink  of  a  shell,  etc.  ;  yet  this  is  not  by  any  means  a  true 
statement  or  description,  as  the  apparent  color  of  the  nerve 

controlled  in  great  part  by  the  surrounding  eye -ground — 
whether  this  is  heavily  pigmented  or  but  slightly  so,  or 
whether  there  is  an  absence  of  pigment,  as  in  the  albino. 
The  student  should  be  ready  to  make  allowances  for  these 
contrasts. 

The  shape  of  the  disc  varies  :  it  may  appear  round,  oval, 
or  even  irregular  in  outline.  Usually  it  is  a  vertical  oval.- 

The  vessels  on  the  disc  which  carry  the  blood  to  and 
from  the  retina  are  not  of  the  same  caliber,  nor  do  they 
have  the  same  curves  and  branches  in  all  eyes  or  in  the 
same  pair  of  eyes.  The  central  artery  may  be  single  or 
double  (if  it  has  branched  in  the  nerve  before  entering  the 
eye),  and  enters  the  eye  at  the  nasal  side  of  the  center  of 
the  disc. 

Approximating  the  central  artery  on  its  temporal  side  is 
the  retinal  vein,  which  may  also  be  double.  The  relative 
normal  proportion  in  size  between  arteries  and  veins  is 

nerally  recognized  as  about  two  to  three.      The  veins  are 
usually  recognized  by  their  larger  size  and  darker  color.  *- 
At  or  near  the  center  of  the  disc  is  often  seen  a  depression, 
I  known  as  the  physiologic   cup  ;  this  may   be  shallow  or 
1  deep  ;  it  may  have  shelving  or  abrupt  edges  ;  it  may  even 
ibe  funnel-shaped. 


94         REFRACTION  AND  HOW  TO  REFRACT. 

At  the  bottom  of  the  cupping  is  frequently  seen  a  gray 
stippling,  the  membrana  cribrosa  ;  openings  in  the  sclera  for 
the  passage  of  the  transparent  optic  nerve-fibers  which  go  to 
form  the  retina.  Surrounding  the  disc  proper  is  often  seen 
a  narrow  white  ring ;  this  is  sclera,  and  is  known  as  the 
^scleral  ring.  Just  outside  of  this  ring  is  frequently  seen  a 
ring  of  pigment ;  this  is  called  the  choroidal  ring.  In  many 
cases  the  choroidal  ring  is  not  complete,  the  pigment  being 
quite  irregular,  or  possibly  there  may  be  just  one  large 
>mass  of  pigment  .to  one  side  of  the  disc ;  this  is  not  patho- 
logic. 

The  retinal  arteries  and  veins,  while  possessing  many 
anomalies,  and  while  occasionally  an  artery  and  vein  are 
seen  to  twine  around  each  other,  usually  pursue  a  uniform 
course  up  and  down  from  the  disc,  and  are  named  accord- 
ingly— i.  c.,  upper  nasal  vein  and  artery  ;  upper  temporal 
'  vein  and  artery  ;  lower,  nasal  artery  and  vein  ;  lower  tem- 
poral artery  and  vein. 

T>  The  retina  itself,  in  health  being  transparent,  is  not  seen. 
The  fovea  centralis,  occupying  the  center  of  the  macular 
region,  is  about  two  discs'  diameter  to  the  temporal  side 
of  the  disc  and  slightly  below  the  horizontal  meridian. 
The  fovea  is  recognized  because  it  is  a  depression,  and  its 
edges  give  a  reflex  ;  it  is  very  small,  and  appears  as  a  bright 
J  spot  one  or  two  mm.  in  diameter.  The  "  macular  region  " 
is  the  part  of  the  eye-ground  immediately  surrounding  the 
fovea ;  it  contains  minute  capillaries,  but  it  is  impossible,  in 
Wealthy  eyes,  to  recognize  them  with  the  ophthalmoscope. 

The  Choroid. — This  is  distinguished  by  the  character  of 

its  circulation,  the  vessels  being  large,  numerous,  and  flat- 

^>tened,  and  without  the  light  streak  which  characterizes  the 

retinal  vessels.    (iPigment  areas  between  the  vessels  are  also 

diagnostic  of  this  tunic)     The  choroidal  circulation  is  best 


OPHTH  A  LMOSCOPE. 


95 


studied  in  the  blond  or  albino,  and  may  be  seen  in  many 
eyes  toward  the  periphery  of  the  eye-ground. 

In  the  foregoing  description  of  the  use  of  the  ophthal- 
moscope, etc.,  it  is  presumed  that  the  instrument  has  been 
used  without  any  lens  in  position,  and  that  the  observer's 
eye  and  the  eye  under  examination  are  healthy  emmetropic 
eyes  with  the  accommodation  at  rest.  Figure  86  shows  the 
position  of  the  light,  L,  the  ophthalmoscope,  the  examiner's 
and  the  examined  eye  under  these  conditions. 

The  divergent  rays  from  the  light  (L)  are  reflected  con- 


FIG.  86. 


vergently  from  the  concave  mirror,  and  focusing  in  the  vitre- 
ous, they  cross'  and  form  an  area  of  illumination  on  the 
retina  at  I  I'.  The  retina,  situated  at  the  principal  focus  of 
the  dioptric  media,  naturally  projects  out  from  its  indi- 
vidual points  rays  of  light  which  are  parallel  as  they  leave 
the  eye ;  some  of  these  pass  through  the  sight-hole  of  the 
mirror  and  meet  upon  the  retina  of  the  observer's  emme- 
tropic eye. 

There   are  twt>  very  important  points  which  must  be 


96 


REFRACTION  AND  HOW  TO  REFRACT. 


considered  when  using  the  ophthalmoscope  in  the  direct 
method  :  one  is  the  direction  which  the  rays  of  light  take 
>  as  they  leave  the  eye  under  examination,  and  the  other  is 
for  the  observer  to  keep  his  own  eye  emmetropic  ;  in  other 
words,  the  observer  wearing  his  correcting  glasses  should 
ji ot  accommodate. 

Figure  87  shows  that  rays  of  light  passing  out  of  an  eye 
divergently  must  be  made  parallel,  so  as  to  focus  upon  the 
surgeon's  own  retina  (emmetropic),  and  to  do  this  it  is 


FIG.  87. — T  B  indicate  points  at  the  edge  of  the  disc  from  which  rays  pass 
out  of  the  eye  divergently  in  the  direction  Tx  B',  1"'  B',  T'  B',  and  being 
received  by  the  observer's  eye,  are  projected  backward,  forming  an  erect 
magnified  image  at  T"  W.  This  image  is  not  so  large  as  that  seen  when 
looking  into  a  myopic  eye.  (Fig.  88.) 

necessary  to  turn  a  plus  lens  in  front  of  the  sight-hole  of 
the  ophthalmoscope  ;  the  strength  of  the  convex  lens  thus 
employed,  other  things  being  normal,  is  the  amount  of  the 
refractive  error  of  the  eye  being  examined. 

Figure  88  shows  rays  of  light  passing  out  of  an  eye 
convergently,  and  to  have  them  parallel,  so  as  to  focus 
upon  his  own  retina  (emmetropic),  it  is  necessary  to  turn  a 
concave  lens  in  front  of  the  sight-hole  of  the  ophthalmo- 


OPHTHALMOSCOPE. 


97 


scope  ;  the  strength  of  the  concave  lens  thus  employed, 
other  things  being  normal,  is  the  amount  of  the  refractive 
error  of  the  eye  under  examination. 

The  Observer's  Accommodation. — It  has  already  been 
stated  that,  when  using  the  ophthalmoscope,  the  observer 
should  wear  any  necessary  correcting  lenses.  If  the  ob- 
server has  a  refractive  error  and  does  not  wear  his  glasses, 
he  must  deduct  this  amount  from  the  lens  used  in  the 
ophthalmoscope.  If  he  has  two  diopters  of  hyperopia 


--B" 


FlG.  88. — T  B  indicate  points  at  the  edge  of  the  disc  from  which  rays  pass 
out  of  the  eye  convergently  in  the  direction  T'  W ,  and,  being  received  by 
the  observer's  eye,  are  projected  backward,  forming  an  erect  magnified 
image  at  T"  B" '.  This  image  is  much  larger  than  that  seen  when  look- 
ing into  the  hyperopic  eye.  (Fig.  87.) 


himself,  and  the  lens  used  in  the  ophthalmoscope  is  plus 
four  diopters,  then  the  eye  under  examination  has  only  two 
|  diopters.     It  is  not  unusual  for  beginners  to  see  the  eye- 
Aground  (disc)   in   hyperopic  eyes  with   a  strong  concave 
lens  ;  this  is  due  to  the  fact  that  they  accommodate.   Prac- 
tice will  overcome  this  habit,  and  it  should  be  mastered  as 
-  soon  as  possible.     There  are  several  ways  of  doing  this : 
one  is  to  begin  the  examination  at  a  distance  of  30  or  40 
9 


98         REFRACTION  AND  HOW  TO  REFRACT. 

cm.  from  the  eye,  with  both  eyes   open,  and   to    gradually 
I  approach  the  eye  as  close  as  3   cm.,  imagining  all  the  time 
/  that  one  is  looking  for  some  remote  point ;  otherwise,  if  one 
begins  the  examination  close  to  the  eye,  and  imagines  he  is 
going  to  see  an  object  about  an  inch  away,  he  will  most 
invariably  accommodate  several   diopters,  with  the  result 
^>  that  he  turns  a  strong  concave  lens  in  front  of  the  sight- 
hole  of  the  ophthalmoscope  to  neutralize  his  accommodation. 
/     This  explains  how  so. many  beginners  diagnose  all  cases 
/of  hyperopia  as  myopia.     An   excellent  way  to  learn  to 
relax  the  accommodation  is  to  practise  reading  fine  print 
at  a  distance  of  about  thirteen  inches  through  a  pair  of 
plus  three  lenses,  placed  before  the  surgeon's  emmetropic 
eyes.     Another  good  way  to  learn  to  relax  the  accommo- 
dation is  to  practise  on  one   of  the  many  schematic  eyes 
found  in  the  shops.     (Fig.  143.) 

Size  of  the  Image  of  the  Eye-ground  (Figs.  87  and 
88). — In  concluding  the  subject  of  the  direct  method  of 
examination  it  may  be  interesting  to  note  the  apparent  size 
of  the  image  of  the  eye-ground,  which,  it  must  be  remem- 
^bered,  is  virtual,  erect,  and  enlarged  ;  in  fact,  it  seems  to 
be  at  some  distance  behind  the  eye,  and  if  the  student  has 
paid  close  attention  to  the  study  of  images  as  formed  by 
convex  lenses,  detailed  in  chapter  i,  he  need  not  have  any 
difficulty  in  appreciating  these  facts. 

The  optic  disc  of  an  emmetropic  eye,  as  seen  through 
ophthalmoscope,  appears  to  be  about  25  mm.  in  diam- 
iter,  and  about  250  mm.  away.  The  retina  of  the  emme- 
tropic eye  is  about  1 5  mm.  from  its  nodal  point ;  then  the 
actual  size  of  the  emmetropic  disc  is  -£/-$  of  25,  or  -f ,  or 
1.5  mm.  ;  then  15  is  to  250  as  1.5  is  to  25,  or  16.6 — the 
magnification,  in  other  words,  when  the  emmetropic  disc  is 
observed,  it  appears  about  16.6  times  larger  than  it  actually  is. 


]u~ 

I 


OPHTHALMOSCOPE. 


99 


The    Indirect    Method  (see   Fig.    89). — Practising  this 
method,  the  observer  sees  a  larger  part  of  the  eye-ground 


FIG.  89. 

at  one  time,  but  it  is  not  so  perfect  in  detail  nor  is  it  mag- 
nified to  the  same  extent  as  in  the  direct  method.  The 
observer  does  not  have  to  get  so  close  to  his  patient,  which 
is  a  decided  advantage  in  some  clinical  cases.  Unfortun- 
ately, as  a  preliminary 
step,  it  is  often  neces- 
sary to  dilate  the  pu- 
pil. In  addition  to  • 
the  ophthalmoscope, 
there  is  also  required 
a  convex  lens  of 
known  strength  and 
large  aperture  ;  the 
one  which  comes  in 
the  case  with  the 
scope  is  usually  too  small  and  too  strong  for  general  use. 
The  writer  prefers  his  plus  13  D.  with  metal  rim  and  conve- 
nient handle,  shown  in  figure  90  (reduced  one-third  in  size). 


FIG.  90. 


IOO        REFRACTION  AND  HOW  /to  KKFKACT. 

_r*    X 

This  is  held  at  about  three  inches  in  front  of  the  eye 
under  examination,  the  observer  resting  his  little  and  ring 
fingers  on  the  temple  of  the  patient.  The  light  may  be 

7  over  the  patient's  head,  or  to  the  side  corresponding  to  the 
eye  under  examination,  the  patient  being  instructed  to  look 
with  both  eyes  open  toward  the  surgeon's  right  ear  when 
ithe  right  eye  is  being  examined,  and  toward  the  surgeon's 
left  ear  when  the  left  eye  is  examined. 

With  a  +4  D.  in  the  ophthalmoscope  held  close  to  his 
eye,  the  surgeon  seats  himself  in  front  of  the  patient  at 

.*>  about  sixteen  inches  distant,  and  reflects  the  light  through 
the  condensing  lens  into  the  patient's  eye,  and  then 
approaches  or  moves  away  from  the  eye  until  he  recog- 
nizes clearly  a  retinal  vessel  or  the  disc  ;  he  must  remem- 
ber, however,  that  he  is  not  looking  into  the  eye,  but  is 
(viewing  an  aerial  image  formed  between  the  convex  lens 
land  the  ophthalmoscope ;  this  image  is  not  only  inverted, 
but  undergoes  lateral  inversion,  so  that  the  right  side 
of  the  disc  becomes  the  left  side  of  the  image,  and  vice 
versa ;  the  upper  side  of  the  disc  becomes  the  lower  side 
of  the  image,  and  vice  versa.  As  the  direct  method  gives 
Ian  erect,  virtual,  and  enlarged  image,  the  indirect  method 
\produces  an  inverted,  real,  and  small  image.  The  principle 
pf  the  direct  method  is  similar  to  a  simple  microscope,  and 
the  indirect  to  a  compound  microscope. 

I      The  size  of  the  image  depends  upon  the  refraction  of 

(  the  eye  and  the  distance  of  the  convex  lens  from  the  eye 
under  examination.  Mn  the  standard  eye  this  is  always  the 
same,  no  matter  how  far  away  from  the  eye  the  convex  lens 
is  held.  )To  estimate  the  size  of  the  image  in  the  standard 
eye,  all  that  is  necessary  to  know  is  the  principal  focal  dis- 
tance of  the  lens  employed  ;  if  a  +13  D.,  then  the  image 
is  formed  at  75  mm.  (three  inches),  and  remembering  that  the 


" 


OPHTHALMOSCOPE.  IOI 

retina  in  the  eye  is  1  5  mm.  back  of  the  nodal  point,  the 
size  of  the  image  will  be  to  the  size  of  the  disc  (if  that  is 
what  is  looked  at)  as  their  respective  distances,  or  as  15  is 
to  75  which  equals  5,  the  magnification. 

The  purpose  of  the  +4  D.  in   the   scope  is  to  take  the 

place  of  the  eve  -piece  in  the  microscope,  and,  therefore,  to 

magnify  the  image  at  the  same  time  it  relieves  the  observer's 

(accommodation.      In  high  myopia  the  -(-4  D.  may  be  dis- 

pensed with. 

The  Luminous  Ophthalmoscope  (Figs.  91  and  92).— 
DcZeng  Patent.  —  This  instrument  is  a  combination  of  the 
Loring  ophthalmoscope  just  described,  with  the  addition  of 
an  electric  light  attachment.  The  mirror  is  somewhat  dif- 
ferent from  the  mirror  on  the  Loring  instrument.  It  is 

f 

plane,  circular  in  form,  14  millimeters  in  diameterfwith  one 
millimeter  of  its  upper  area  cut  off  horizontally^-  The  flat 
top  edge  of  the  mirror  is  on  a  level  with  the  lower  edge  of 
the  sight-hole  in  the  ophthalmoscope.  The  observer  in 
looking  through  the  sight-hole  always  looks  over  the  mirror 
and  never  through  it.  J  The  mirror  is  placed  at  an  angle  of 
43  degrees.  The  handle  of  the  instrument  carries  the  elec- 
tric wires  to  a  small  lamp,  and  between  the  lamp  and  the 
mirror  is  placed  a  very  strong  convex  lens.  The  rays  of 
light  from  the  filament  falling  upon  the  convex  lens  are 
refracted  very  coiivergently,  and  after  reflection  from  the 
mirror  converge  to  a  point  one  inch  distant.  (See  Fig.  92.) 
This  instrument  is  ideal  for  both  the  direct  and  indirect 
method  and  has  the  following  points  of  merit:  The  mirror 
and  light  arc  stationary,  thus  giving  the  observer  any 
liberty  of  movement  necessary  without  any  loss  of  reflec- 
tion from  the  mirror;  the  mirror  never  requires  any  tilting; 
the  brilliancy  or  intensity  of  the  illumination  at  the  ftindus, 
by  virtue  of  the  light  being  so  close  to  the  mirror,  far  ex- 


IO2 


REFRACTION  AND  HOW  TO  REFRACT. 


ceeds  that  of  the  nonluminous  instrument;  for  the    same 
reason  the  size  of  the  retinal  illumination  is  made  about  five 


FIG.  91.  FIG.  92. 

FIGS.  91  and  92. — DeZeng  Luminous  Ophthalmoscope.     Two-thirds  size. 

times  larger  than  that  by  the  old  style  instrument.    The  heat 
from  the  electric  lamp  (2^4  volts,  ^  ampere)  is  infinitesimal. 


CHAPTER  IV. 

EMMETROPIA.— HYPEROPIA.— MYOPIA. 

KMMETROPIA. 

Emmetropia  ('•>,  "in";  plrpw,  "measure";  fy1, 
"  eye  ")  literally  means  an  eye  in  measure,  or  an  eye  which 
has  reached  that  stage  of  development  where  parallel  rays 
of  light  will  be  focused  on  its  retina  without  any  effort  of 
accommodation.  As  the  emmetropic  eye  is  the  ophthal- 
mologist's ideal  unit  of  measurement  or  goal  in  refraction, 
the  beginner  should  know  this  form  of  eye  thoroughly,  so 
that  he  may  recognize  any  departure  from  this  standard 
condition.  The  emmetropic  eye  may  be  described  in  vari- 
ous ways,  and  while  these  descriptions  may  appear  like 
repetitions,  they  are  given  for  purposes  of  illustration  : 
,  The  standard  or  schematic  eye:  Authorities  differ  some- 
what in  the  exact  measurements  of  a  schematic  eye,  but 
the  one  suggested  by  Helm- 
holtz  is  certainly  worthy  of 
careful  consideration.  (See 

P-  59-) 

An  emmetropic  eye  is  one 

which,    in     a    state     of    rest 

(without  an}-  effort  of  accom-  FIG.  93. 

modation   whatever),  receives 

parallel   rays   of  light  exactly  at  a  focus    upon   its  fovea. 

(See  Fig.  93.) 

An  emmetropic  eye,  therefore,  is  one  which,  in  state  of 
rest,  emits  parallel  rays  of  light.      (See  Fig.  93.) 

103 


104 


REFRACTION  AND  HOW  TO  REFRACT. 

An  emmetropic  eye  is  one 
whose  fovea  is  situated  exactly 
at  the  principal  focus  of  its  re- 
fractive system.  (See  Fig.  93.) 

*l  An  emmetropic  eye  is  one 
the  vision  of  which,  in  a  state 
of  rest,  is  adapted  for  infinity. 

\  '  An  emmetropic  eye  is  one 
[which  has  its  near  point  consist- 
ent with  its  age.  (See  p.  70.) 

An  emmetropic  eye  is  one 
which  does  riot  develop  pres- 

7byopic  symptoms  until  forty- 
five  or  fifty  years  of  age.  (See 
p.  272.) 

\  An  emmetropic  eye,  in  con- 
tradistinction to  a  myopic  eye 
(see  p.  115),  is  spoken  of  as 
ia  healtKy  eye,  or  one  which 

A  shows  the  least  amount  of  irri- 
tation in  its  choroid  and  retina. 
Because  we  refer  to  Hclm- 
holtz's  schematic  eye  as  an  em- 
metropic eye,  it  will  not  do  to 
say  that  all  eyes  that  measure 
just  23  mm.  in  their  antero- 
posterior  diameter  are  emme- 
tropic (Fig.  94);  for  while  an 


FIG.  94. — I.  Emmetropia.  2.  Myopia 
due  to  a  strong  lens.  3.  Hyperopia 
due  to  a  weak  lens.  4.  Myopia  due 
to  a  short  radius  of  curvature  of  cornea. 
5.  Hyperopia  due  to  a  long  radius  of 
curvature  of  cornea.  The  anteropos- 
terior  diameter  of  all  these  eyes  is  just 
23  mm. 


AMETROPIA.  IO5 

eye  maybe  just  23  mm.  in  length,  it  may  have  its  refractive 
system  stronger  or  weaker  than  is  consistent  \vith  its  length, 
making  it,.  if  stronger,  a  myopic  or  long  eye,  and,  if  weaker, 
a  short  or  hyperopic  eye.  An  eye,  to  be  emmetropic, 
therefore,  no  matter  what  its  length,  must  have  its  refractive 
apparatus  of  just  such  strength  that,  in  a  state  of  rest,  the 
principal  focus  will  coincide  exactly  with  the  cones  at  the 
fovea.  (Fig.  93.) 

cu  AMETROPIA. 

Ametropia  (a  priv.  ;  /^r/>..>,  "a  measure"  ;  o<?>s,  "sight") 
literally  means  "  an  eye  out  of  measure."  An  ametropic 
eye  is  one  which,  in  a  state  of  rest,  docs  not  form  a  distinct 
image  of  distant  objects  upon  its  retina.  An  ametropic 
eye  may  be  defined  as  one  which,  in  a  state  of  rest,  does 
not  focus  parallel  rays  of  light  upon  its  fovea.  fAn  eye 
which  is  not  emmetropic  is  ametropic.J)  There  are  two 
of  ametropia  —  axial  and  curvature  ametropia. 

Axial  ametropia  is  the  condition  in  which  the  dioptric 
apparatus  refracts  equally  in  all  meridians,  but  the  retina 
of  the  eye,  when  at  rest,  is  either  closer  to,  or  further  away 
from,  the  nodal  point  than  the  principal  focus.  (See  Figs. 
95  and  97.)  The  refraction  is  measured  on  the  length  of 
the  anteropostcrior  axis  of  the  eye  ;  hence  its  name,  axial 
metropia. 

Curvature  ametropia,  in  contradistinction  to  axial  ame- 
tropia,  is  the  condition  in  which  the  dioptric  apparatus  does 
not  refract  equally  in  all  meridians,  and  with  the  result  that 
there  is  no  focusing  of  all  the  rays  at  any  one  point  ;  or 
curvature  ametropia  may  be  considered  as  that  condition 
in  which  parallel  rays  of  light  entering  an  eye  have  two 
focal  planes  for  two  principal  meridians  at  right  angles  to 
each  other.  Curvature  ametropia  is  commonly  spoken  of 
as  astigmatism.  (See  Chap,  v.) 


|  ____ 


IO6        REFRACTION  AND  HOW  TO  REFRACT. 

Varieties  of  Axial  Ametropia. — Axial  ametropia  is  of 
two  forms  :  one  in  which  the  eye  has  its  fovea  closer  to  the 
Idioptric  apparatus  than  its  principal  focus  (see   Fig.   95), 
|  known   as  the  hyperopic,  short,  or  flat  eye  ;   and  the  other 
form  of  the  eye  in  which  the  fovea  is  further  away  than  its 
principal  focus,  known  as  the  myopic  or  long  eye.      (See 
Fig.  97.) 

HYPEROPIA  OR  HYPERMETROPIA. 

Hyperopia  (t>-lp,  "over";  &?,  "eye")  literally  means 
an  eye  which  does  not  equal  the  standard  condition,  or  an 
eye  which  is  less  than  the  standard  measurement.  Hyper- 
opia is  often  abbreviated  H.  £  About  twenty  per  cent, 
of  all  eyes  have  simple  hyperopiaj)  The  hyperopic  eye  is 
spoken  of  as  far-sighted,  and  the  condition  as  one  of  far- 
sightedness. The  hyperopic  eye  may  be  described  in  many 
different  ways  : 

1.  The  "  natural  eye,"  or  "the  eye  of  nature."  tf>  X^n^*** 

2.  The  "short  eye."     This  term  is  used  on  account  of 
its  fovea  lying  closer  to  the  dioptric  apparatus  than  the 
principal  focus. 

•      3.   Parallel  rays  of  light  passing  into  a  hyperopic  eye  in 
P  a  state  of  rest  fall  upon  its  retina  or  fovea  before  they  focus. 
I  (See  Fig.  7 1 .) 

4.  Rays  of  light  from  the  fovea  of  a  hyperopic  eye  in  a 
state  of  rest  pass   out   divergently  (see  Fig.  95),  and  the 
condition  is  equivalent  to  a  convex  lens  refracting  rays  of1 
light  which  proceed  from  a  point  close'r  to  the  lens  than  its 
principal  focus.     (See  Fig.  39.) 

5.  A  hyperopic  eye  is  one  which,  in  a  state  of  rest,  can 
receive  only  convergent  rays   of  light  at  a  focus  upon  its 
fovea  (Fig.  95)  ;  therefore,  to  repeat :  the  hyperopic  eye,  in 
a  state  of  rest,  emits  divergent  rays  and  receives  convergent 
rays  at  a  focus  upon  its  fovea. 


HYPEROPIA.  IO7 

6.  As  convergent  rays  are  not  found  in  nature,  and  are, 
therefore,  artificial,  a  hyperopic  eye  is  one  which,  in  a  state 
of  rest,  requires  a  convex  lens  to  focus  parallel  rays  of  light 
on  its  fovea.      (See  Fig.  96.) 

7.  A  hyperopic  eye  is  one  which  must^accommodate  for 
infinity,  and,  in  fact,  for  all  distances  ;  in  other  words,  a 
hyperopic    eye   in    use    is   in   a    constant   state   of  accom- 
modation. 

8.  A  hyperopic  eye  having  to  use  some  of  its  accommo- 
dative power   for   infinity,   must,  in  consequence,   have  its 
near  point  removed  beyond   that  of  an   emmetropic  eye  of 
corresponding  age.      (See  p.   72.) 

9.  From  the  description  contained  in  3,  it  follows  that 


FIG.  95.  FIG.  96. 

the  far  point  of  a  hyperopic  eye  in  a  state  of  rest  is  negative 
( — ),  and  is  found  by  projecting  the  divergent  rays  back- 
ward to  a  point  behind  the  retina.  (See  Fig.  95.) 

10.  From  the  description  contained  in  6,  and  the  descrip- 
tion of  accommodation  on  page  67,  it  is  natural  to  find  the 

ir. 

retina  and  choroid  of  many  hyperopic  eyes  in  a  state  of 
irritation. 

11.  From  the  description  contained  in  6  and  7,  and  on 
page  273,  it  follows  that  symptoms  of  presbyopia  manifest 
themselves  earlier  in  hyperopic  than  in  any  other  form  of 
eyes. 

12.  From  the  description  contained  in  5, — and  this  may 


IO8        REFRACTION  AND  HOW  TO  REFRACT. 

appear  like  repetition),  it  follows  that  a  hyperopic  eye  will 
'"*  accept  a  plus  glass  for  distant  vision.     (See  Fig.  96.) 

13.   From  6  it  is  evident  that  the  circular  fibers  of  the 

ciliary  muscle  must  become  highly  developed  ;  much  more 

so  than  the  longitudinal  fibers.      Microscopically,  a  section 

of  the  ciliary  muscle  on  this  account  will  bear  evidence  of 

>iS  ,/^th(f  character  of  the  eye  from  which  it  came. 

Causes  of  Hyperopia. — It  is  a  well-known  fact  that  the 
^eyes  of  the  new-born  are,  with  comparatively  few  excep- 
tions, hyperopic  ;  such  eyes  are  supposed  to  grow  in  their 
anteroposterior  diameter,  and  at  adolescence  to  reach  that 
stage  of  development  called  emmetropia.  It  is  also  a  well- 
known  fact  that  this  ideal  condition  of  emmetropia  is  very 
rarely  attained,  the  length  of  the  eyeball  not  increasing  in 
proportion  to  the  strength  of  its  refractive  system. 

Eyes  may  approximate  the  emmetropic  condition,  but 
very  seldom  remain  so,  passing  into  the  condition  in  which 
the  fovea  lies  beyond  the  principal  focus,  becoming  what  is 
known  as  long,  or  myopic. 

A  standard  eye  may  be  made  hyperopic  by  removing 
its  lens  ;  the  condition  following  cataract  extraction.  (See 
Fig.  174.) 

/     An  eye  may  possibly  become  hyperopic  in  old  age,  from 
flattening  of  the  lens  due  to  sclerosis  of  its  fibers. 

Any  disease  which  will  cause  a  flattening  of  the  cornea 
in  a  standard  eye  will  produce  hyperopia. 
^>A.  diminution  in  the   index  of  refraction   of  the  media 
of  the  standard  eye  will  produce  hypcropial^^Aist/Tfy 

Subdivisions  of  Hyperopia. — For  purposes  of  study 
hyperopia  has  been  divided  into  six  classes  or  forms  : 

I.  Facultative  hyperopia  (abbreviated  Hf.)  is  a  condi- 
tion of  the  eye  in  which  the  patient  can  overcome  the  error 
by  using  his  accommodation.  It  is  a  condition  of  early 


H\TEROPIA.  IO9 

t 

life,  and  is  voluntary.      The  patient  can  see  clearly,  with  or 
without  a  convex  glass. 

2.  Absolute  hyperopia  (abbreviated  Ha.). — This  is  hy- 
Iperopia  that   can   not  be  overcome  by  the  accommodative 
I  effort.      It  is  generally  a  condition  of  old  age,  and  is  invol- 
untary ;  facultative  hyperopia  in  youth  becomes  absolute  in 
old  age.  ^  Old  age,  in  fact,  may  develop  each  variety  except 
latent  hyperopia.     Absolute  hyperopia  exists  whenever  the 
defect  is  of  so  high  a  degree  that  it  can  not  be  overcome 
by  the  accommodation  or  when  the  accommodative  power 
itself  is  gone. 

3.  Relative  hyperopia  (abbreviated    Hr.)  is  where    ac- 
commodation is  assisted  in  its  efforts  by  the  internal  recti 
muscles  ;  in  other  words,  the  eyes  squint  inward. 

4.  Manifest    hyperopia    (abbreviated    Hm.)  is    repre- 
sented by  the  strongest  convex  lens  through  which  an  eye 
can  maintain  distinct  distant  vision.     Manifest  hyperopia, 
therefore,  includes  facultative  and  absolute. 

5.  Latent  hyperopia  (abbreviated   HI.)  is    the  amount 
\of  hyperopia  which  an  eye   retains  when   a  plus   lens  is 

''  (placed  in  front  of  it.  /Or  latent  hyperopia  is  the  difference 
between  the  manifest  hyperopia  and  that  lens  which  an  eye 
would  select  if  its  accommodation  was  put  at  rest  with  a 
cycloplegic  (atropin)J  For  example,  an  eye  accepts  a 
-f- 1.25  S.  as  its  manifest  H.,  and,  when  atropin  is  instilled, 
would  accept  +2.75  S.  for  the  same  distant  vision  ;  then 
the  difference  between  the  manifest  +1-25  S.  and  +2.75 
S.  (the  total)  is  +1.50  S.,  which  is  the  latent  hyper- 
opia. 

6.  Total  hyperopia  (abbreviated  Ht.)  is  the  full  amount 
-->  of  the  hyperopia  ;  or  is  represented  by  the  strongest  glass 

which  an  eye  will  accept,  and  have  clear,  distinct  vision 


when  in  a  state  of  rest.     &/v 


IIO        REFRACTION  AND  HOW  TO  REFRACT. 

Symptoms  and  Signs  of  Hyperopia.- — These  are  many 
and  various  ;  the  principal  one,  however,  and  the  one  that 
^generally  causes  the  patient  to  seek  relief,  is  headache. 
Headache  caused  by  the  eyes  is  usually  frontal,  and  is 
denominated  "  brow  ache "  ;  it  may  be  frontotemporal  ; 
the  pain  or  discomfort  starting  in  or  back  of  the  eyes  may 
extend  to  the  occiput  or  all  over  the  head,  and  be  accom- 
panied with  all  kinds  of  nervous  manifestations.  The  most 
characteristic  distinguishing  feature  of  ocular  headache  is 
that  it  comes  on  while  using  the  eyes,  and  gradually  grows 
worse  as  the  use  of  the  eyes  is  persisted  in  ;  and,  likewise, 
the  headache  gradually  ceases  after  a  few  minutes'  or  hours' 
rest  of  the  eyes.  Vertex  headache,  or  a  feeling  of  weight 
on  the  top  of  the  head,  has  been  preempted  by  the  gyne- 
cologist, and  is  not  usually  classed  as  ocular.  The  ciliary 
muscle  being  the  prime  factor  in  causing  the  headach 
the  writer  feels  justified  in  calling  it  the  "  headache  mus- 
cle." "  Sick  headaches  "  are  largely  due  to  eye-strain. 
Various  functional  disorders,  such  as  dyspepsia,  constipa- 
tion, biliousness,  lithemia,  chorea,  convulsions,  epileptoid 
diseases,  hysteria,  melancholia,  etc.,  are,  according  to  some 
few  authorities,  attributable  to  this  condition.  See  Asthen- 
opia,  page  219. 

Blepharitis    marginalis,    styes,     and     conjunctivitis    are 

frequently  present,  and    in    truth    the    hyperopic    eye    on 

^  this    account   can    often   be    diagnosed    in    public   outside 

of  the  surgeon's  office.      A  feeling  as  of  sand  in  the  eyes, 

ocular  pains  or  postocular    discomfort,  a  dryness  of  the 

iids,  as  if  they  would  stick  to  the  eyeballs,  are  common 

complaints,  and  part  of  the  conjunctivitis.     Other  patients 

have  their  eyes  filling  with  tears  (epiphora)  as  soon  as  they 

-jr  begin  reading,  etc.     A  drowsiness  or  desire  to  sleep  often 

comes  on  after  or  during  forced  accommodation. 


r 


/ 


HYPEROPIA.  I  I  I 

Congestion  of  the  choroid  and  retina,  as  evidenced  by  the 
ophthalmoscope,  often  go  together  with  the  blepharitis  and 
conjunctivitis. 

The  patient  complains  that  the  print  blurs  or  becomes  dim 
after  reading,  and  this  is  especially  apt  to  occur  by  artificial 
light.  When  the  "blur"  comes  on,  he  has  to  stop  and 
rub  his  eyes  or  bathe  them  ;  and  then,  with  additional  light, 
he  is  able  to  continue  the  reading  for  a  short  time  longer, 
when  the  blur  again  returns  and  the  effort  must  be  given  up. 
Strong  light  stimulates  the  accommodation.  The  "  hyper- 
>  opic  blur"  is  nothing  more  or  less  than  a  relaxation  of  the 
accommodation. 

In  children  hyperopia  sometimes  simulates  myopia,  from 
the  fact  that  the  child  in  reading  holds  the  print  very  close 
to  the  eyes.  He  does  this  in  order  to  get  a  larger  retinal 
image  and  to  relieve  his  accommodation  ;  the  retinal  image 
is  not  clear,  and  the  child  has  to  read  slowly ;  the  retinal 
image  is  composed  mostly  of  diffusion  circles.  The  child 
holds  the  print  close  to  his  eyes  to  avoid  using  his  total 
accommodation,  which  he  might  have  to  do  if  he  held  the 
print  at  a  respectable  distance. 

He  also  calls  into  play  the  orbicularis  palpebrarum,  and 
narrows  the  palpebral  fissure,  looking  through  a  stenopeic 

slit,  as  it  were.     These  cases  of  simulated  myopia  can  be 

7       • 
jquickly  diagnosed  by  : 

I.   The  narrowr  palpebral  fissure  during  the  act  of  reading, 
and  reading  very  slowly,  as  each  letter  has  to  be  studied. 
7  2.  The  fact  that  very  few  children  have  myopia. 

3.  The    comparatively    good    distant  vision,  as  a    rule, 
which   myopes   never  have,  unless  the  myopia  is  of  very 
small  amount. 

4.  The  ophthalmoscope. 

The  beginner  in  ophthalmology  should  be  on  his  guard 


I  1 2        REFRACTION  AND  HOW  TO  REFRACT. 

for  these  "pseudo-myopias,"  and  not  be  guilty  of  putting 
concave  lenses  on  hyperopic  eyes. 

Diagnosis  of  Hyperopia. — This  form  of  ametropia  may 
be  recognized  in  many  ways  : 

1.  Blepharitis  marginalis,  if  present,  is  generally  due  to 
hyperopia. 

2.  Hyperopic  eyes  are  said  to  be  small,  and  to  have 
.  small    pupils,    which    facts    are   generally    confirmed ;    but 

myopic  eyes  sometimes  appear  small,  and  have  small  pupils 
also. 

3.  A  narrow  face  and  short  interpupillary  distance  are 
quite  indicative  of  hyperopia,  but   these  indexes  are  not 
infallible. 

4.  A  child  with  one  eye  turned  inward  toward  the  nose 


, 


'  (convergent  squint)  has  hyperopic  eyes,  as  a  rule ;  the 
hyperopia  generally  not  being  of  the  same  amount  in  the 
two  eyes,  the  squinting  eye  usually  being  the  more 
hyperopic. 

5.   It  has  been  authoritatively  stated  that  light-colored 
irises  are  seen  in  hyperopic  eyes  and  dark  irises  are  to  be 

\found  in  myopic  eyes,  and  yet  this  is  not  always  correct. 
German  students,  with  their  blue  irises,  will  average  from 
50  per  cent,  to  60  per  cent,  of  myopia. 

I     6.   Hyperopic  eyes,  with  few  exceptions,  have  excellent 

/  distant  vision  :  often  ~,  or  even  better.  The  student  should 
be  on  his  guard  for  this,  and  not  imagine,  because  a 
patient  has  •—  vision,  that  he  is  emmetropic  ;  on  the  con- 
trary, hyperopic  eyes  accommodate  for  distance,  and  obtain 
this  acute  vision  by  effort. 

7.  The  patient  gives  a  history  of  accommodative  asthe- 
nopia,  with   or  without  headaches   coming    on  during  or 
after  the  use  of  the  eyes. 

8.  The   distant  vision  of  a  hyperopic  eye  may  remain 


MYOPIA.  -II3 

unchanged  or   may  be  improved   with  the    addition  of  a 

convex  lens,  which  latter  would  be   impossible   in   emme-      / 

i  ">/  A^^Lti^i*   ->t^  &^r 

tropia  and  myopia.       '  '^»C~^'t^  r*4-   -7^^  t-*c*  * 

9.  The  near  point  of  a  hyperopic  eye  without  glasses  lies 
beyond  that  of  an  emmetropic  eye  for  a  corresponding  age. 

10.  A  hyperopic  eye  can  see  fine  print  clearly  through  a 
convex  lens  at  a  greater  distance  than  its  principal  focus, 
which  would  not  be  the  case  in  any  other  form  of  eye. 

Other  tests  for  determining  hyperopia  are  with  (i  i)  the 
ophthalmoscope,  (12)  the  retinoscope,  (13)  Scheiner's  test, 
(14)  Thomson's  ametromcter,  and  (15)  the  cobalt-blue 
glass  test,  commonly  spoken  of  as  the  chromo-aberration 
test.  These  tests  are  described  in  the  text' 

MYOPIA.  ..,,£ 

Myopia  (v^b,    "to  close";   a><}>,    "eye")  means,   liter- 
ally, "to  close  the  eye,"  and  this  origin  of  the  name  has 
arisen  from  the   fact  that  many  long  eyes  (myopic)  squint 
the  eyelids  together  when   they  endeavor  to  see  beyond 
their  far  pointNBrachymetropia  is  another  name  occasionally 
mentioned  for  the  same  kind  of  eye.     Myopia  is  abbreviated 
About  1.5  per  cent,  of  all  eyes   have  simple  myopia. 
The  myopic  eye  is  spoken 
of  as  near-sighted,  and  the 
condition  as   one  of  near- 
sightedness.     The  myopic 
eye    may  be   described   in 
many  different  ways  : 

1.  The  long  eve.     The 

•  •      -  r  IG.  97- 

origin     of     this     name    is 

purely  anatomic,  thoffovea  lying  beyond  the  principal  focus 

of  the  refracting  system.      (See  Fig.  97.) 

2.  Parallel  rays  of  light  entering  a  myopic  eye  focus  in 


114 


REFRACTION    AND    HOW    TO    REFRACT. 


the  vitreous  humor  before  they  can  reacli  the  fovea.     (See 

Fig-  97-) 

3.    Rays  of  light  from  the  fovea  of  a  myopic  eye  pass  out 

of  the  eye  convergently  (see  Figs.  72  and  98),  focusing  at 


FIG.  98. 

some  point  inside  of  infinity.     The  refractive  condition  of 
ja  myopic  eye  is   similar  or  equivalent  to  a  convex  lens 
-^  Irefracting   rays  of  light   which   proceed   from   some  point 
(further  away  than  its  principal  focus.     (See  Fig.  37.)     The 
/nearer  the  emergent  rays  of  light  focus  to  the   eye  (in  a 
state  of  repose),  the  longer  the  eye  ;  and  the  further  away 
the  emergent  rays  focus  from  the  eye,  the  nearer  the  eye 
approaches  to  emmetropia,  or  normal  length. 

4.  A  myopic   eye  is   one   which  receives   rays  of  light 
which  diverge  from  some  point  closer  than  six  meters  at  a 

focus  on  its  fovea  and  which 
emits  convergent  rays.  (See 
Fig-  37>  ar>d  also  description 
of  conjugate  foci.) 

5.  As  parallel  rays  can  not 
focus  on  the  fovea  of  a 
myopic  eye,  it  is  necessary  to 
give  parallel  rays  entering  the 

eye  a  certain  amount  of  divergence,  so  as  to  place  the  focus 
at  the  fovea ;  and  to  accomplish  this,  a  concave  lens  must 
be  used.  (See  Fig.  99.)  A  myopic  eye,  therefore,  is  one 


FIG.  99. 


MYOPIA.  115 

which  requires  a  concave   lens  to  improve  distant  vision. 
(See  Fig.  99.) 

/6.  A  myopic  eye  is  one  whose  distant  vision  is  made 
worse  by  the  addition  of  a  convex  lens. 

7.  A  myopic  eye  is  one  which  does  not  accommodate  for 

,.  t 
distance. 

8.  A   myopic   eye  having  a  refracting  system  stronger 
than  is  consistent  with   its   length,  or  vice   versa,   greater 
length  than  is  consistent  with  its  dioptric  system,  naturally 
does  not  use  any  accommodation  except  for  points  inside  of 
its  punctum   remotum,  and  with  the   result  that  its  ampli- 
tude of  accommodation  is  used  near  by  ^consequently,  a 
myopic  eye  is  one  which  has  a  near  point  closer  than  an  — , 
emmetropic  eye  of  corresponding  age. )  (See  p.  73.) 

9.  From  the  description  contained  in  3  it  follows  that  the 
far  point  of  a  myopic  eye  is  positive  (jj^).  £  j^i  vry( 

10.  From  the  description  contained  in  3  and  7,  it  also 
follows  that  the  myopic  eye  does  not  develop  presbyopic 
symptoms  until  late  in  life. 

1 1.  From  6  and  9   it  follows  that  the  circular  fibers  of 
the  ciliary  muscle  are    not  used  to  the  same  extent  in  a 
myopic  eye   as    in  the   emmetropic  and  especially  in  the 
hyperopic   eye.       Microscopically,    a    section    of  a   ciliary 
muscle  on  this  account  will  bear  evidence  of  the  character/ 

|  of  the  eye  from  which   it  came,  and  have  the  longitudinal 
'   fibers  more  in  evidence.      In  some  very  long  myopic  eyes 
there  may  not  be  any  circular  fibers  recognized. 

i  2.    I -AX-S  in  which  the  myopia  is  progressive  are  spoken 
of  as  "  sick  eyes."/  "r Ln^*+*-^*-*-<~   >lE>2<*-*<^-  P*J*-*-~*^' 

"  '  ~  ^ 

Causes  of  Myopia. — Any  disease  or  injury  which  will  / 
so  alter  the  refracting  system  of  an  eye  that  parallel  rays  /• 
must  focus  in  front  of  the  fovea  will  produce  the  form  of  <^r*-**^-^ 
eye  known  as  long  or  myopic.     This  may  be  brought  about    > 


in  different  ways  :  A  shortening  in  the  radius  of  curvature 
of  the  cornea,  such  as  comes  with  conic  cornea  and  staphy- 

loma  of  the  cornea  ;    an  increase  in  the   refractive  power 

^  &* 

of  the  lens  from  swelling,  as  often  precedes  cataract,  and 

is  spoken  of  as  "false  second  sight ; /cyclitis  and  irido- 
cyclitis,  which  diseases  cause-  a  relaxation  of  the  lens 
ligament,  allowing  the  lens  to  assume  a  greater  convexitvj 
or  ciliary  spasm  may  produce  temporarily  the  same  con- 
dition. 

Technically,  however,  myopia  is  quite  universally  under- 
stood to  mean  a  permanent  elongation  of  the  visual  axis 
of  the  eye  beyond  the  principal  focus  of  its  refracting 
system. 

Heredity  is   certainly  a  predisposing  factor  to  myopia, 
but  this  does  not  mean  that  the   babe   is   necessarily  born 
with  long  eyes.     On  the  contrary,  the  eye  is  very  likely 
hyperopic  at  birth,  and/what  the  child  may  inherit  is  weak 
(eye  tunics,)    Such  eyes,  when  placed  under  strain  or  what 
xfto  them  is  overuse,  soon    become  elongated.     This    may 
also  be  brought  about  or  assisted  by  poor  hygienic  surround- 
ings, poor  health,  or  develop  after  an  attack  of   typhoid 
or  one  of  the~eruptive  fevers. 

.T^Three  causes  for  the  elongation  of  eyes  have  been  brought 
forward  by  able  authorities  and  expounded  as  theories,  any 
one  of  which,  or  all  three,  may  appear  conspicuously  in  in- 
dividual cases. 

1.  Anatomically,  the  size  of  the  orbit  and  the  broad 
face  give  a  long  interpupillary  distance  and  cause  excessive 
convergence  (turning  inward  of  the  eyes)  when  the  eyes  fix 
at  the  near  point. 

2.  Mechanically,    when    the   eyes   are   far   apart   and 
attempt  to  converge,  the  external  recti  muscles  press  upon 
the  outer  side  of  the  globes,  flattening  the  eyes  laterally, 


MYOPIA.  1 1 7 

with  the  result  that  the  point  of  least  resistance  for  the 
compressed  contents  of  the  globes  is  at  the  posterior  pole 
of  the  eye,  and  here  it  is  that  the  pressure  shows  itself,  by 
an  elongation  of  the  eye  backward  in  its  anteroposterior 
diameter.  ^This  combination  of  the  anatomic  and  mechanic 
theories  may  explain  in  great  part  the  presence  of  myopia 
in  the  average  German  student  or  any  broad-faced  indi- 
vidual.^ 

3.  The  inflammatory  theory  is  that  a  low  grade  of 
inflammation  attacks  the  tunics  of  the  eye,  especially  at 
the  posterior  pole,,  and  is  spoken  of  as  macular  chor- 

(oiditis  ;  this  is  brought  about  by  faulty  use  of  the  eyes, 
in  the  school  or  in  the  home,  in  a  poor  light  or  too 
glaring  a  light  improperly  placed,  or  by  using  the  eyes 
with  the  head  bent  over  the  work  so  that  the  return  circu- 
lation from  the  retina  and  choroid  is  interfered  with./  This 
inflammation  or  congestion  of  the  tunics  of  the  eye  may 
be  primary  in  itself  or  secondary  to  the  anatomic  and 
mechanic  causes.  Be  this  as  it  may,  the  conditions  exist, 
and  go  to  show  more  and  more  that  myopia  is  actually 
acquired  and  not  per  sc  congenital.  "  The  inherited  con- 
genital anomalies  of  refraction,  particularly  astigmatism,  are 
responsible  for  the  myopic  eye,  by  virtue  of  the  pathologic 
changes  they  occasion  in  hard-worked  eyes  rather  than  any 
inherited  predisposition  to  disease."  (Risley,  "School 
1  Ivgiene.") 

Symptoms  and  Signs  of  Myopia. — While  the  myope 
may  complain  of  headache  and  symptoms  of  accommodative 

/asthenopia,  yet  the  principal  visual   complaint  will   be  the 

,  inability  to  see  objects  distinctly  which  lie  beyond  the  far 

point.     The  myope's  world  of  clear  vision  is  limited  to  the 

distance  of  the  far  point,  where  the  rays  of  light  leaving  his 

eye  come  to  a  focus.      Every  object  situated  beyond  the  far 


Il8        REFRACTION  AND  HOW  TO  REFRACT. 

point  is  blurred  and  indistinct,  and  the  further  the  object 
from  the  far  point,  the  more  indistinct  it  becomes.  The 
myopic  child  at  school  soon  ranks  high  in  the  class,  is  fond 
of  study,  of  books,  music,  or  needlework,  according  to  the 
sex.  The  myope,  in  other  words,  is  usually  literary  in 
taste.  Myopes  avoid  out-of-door  sports,  such  as  foot-ball, 
base-ball,  golf,  etc. 

Diagnosis  of  Myopia. — This  form  of  ametropia  may  be 
recognized  in  various  ways  : 

1.  The  prominent  eyeball.     This  is  not  a  positive  sign  of 
myopia,  though  this  and  other  signs  are  mentioned  for  the 
reason  that  they  are  often  present  in  the  myopic  condition. 

2.  The  broaa  face  and  (3)  long  interpupillary  distance 
are  quite  significant  of  myopia,  and  yet  the  broadest  face 
with    longest   interpupillary  distance  the  writer  ever  saw 
was  in  a  hyperopic  subject. 

4.  Divergent  squint   usually  indicates  myopia,  and  this 
condition  is  often  brought  about  by  an  inability  to  converge, 

'  or  one  eye  may  be  more  myopic  than  its  fellow,  with  the 
result  that  the  more  myopic  eye  turns  out  and  soon  be- 
comes amblyopic. 

5.  (It  has  been  stated  that  myopic  eyes  usually  have 
dark-colored  irises>(  but  this  is  often  a  fallacy,  as  is  only  too 
evident  in  the  German  student  with  his  blue  iris. 

The  foregoing  are  but  signs  of  myopia,  and  are  recog- 
nized by  inspection  ;  they  should  be  looked  for  and  care- 
fully estimated,  and  each  given  its  due  consideration. 
'Subjective  and  objective  symptoms  are  the  true  tests  of 
myopia,  and  are  as  follows  : 

6^>Poor  distant  vision  ;  inability  to  see  numbers  on  the 
houses  across  the  street  or  on  the  same  side  of  the  street ; 
history  of  passing  friends  without  speaking  to  them.  The 
myope  enjoys  close  work  and  takes  little  or  no  interest  in 


MYOPIA.  I  19 

sports.     A  history,  in  other  words,  that  is  in  keeping  with 
a  vision  of  short  range. 

7.-^Good  near  vision  ;  ability  to  see  the  finest  print  or  to 
thread  the  finest  needle  or  do  the  finest  embroidery. 

8.  The  near  point  is  closer  than  that  of  an  emmetropic 
eye  of  corresponding  age.  (See  p.  73.) 
I  9.  Distant  vision  is  made  worse  by  the  addition  of  a 
convex  lens.  The  writer  prefers  to  teach  the  diagnosis  of 
myopia  in  this  way,  and  not  to  say  that  a  concave  lens 
will  improve  distant  vision  ;  of  course  it  will,  but  he  does 
not  want  the  student  to  put  concave  lenses  before  the 
eye  of  the  young  "pseudo-myope,"  referred  to  under 
Hyperopia. 

io.(ihe  far  point  is  brought  nearer  by  the  addition  of  a 
convex  lens./  Objective  methods  of  determining  myopia 
are  by  means  of  the — 

11.  Ophthalmoscope. 

12.  Retinoscope. 

13.  Schemer's  method. 

14.  Thomson's  ametrometer. 

15.  Chromo-aberration  test. 

Direct  Ophthalmoscopy  in  Axial  Ametropia. — Pro- 
ficiency in  this  method  comes  only  by  perseverance  and 
long  practice.  It  should  not  be  employed  to  the  exclusion 
of  other  and  more  exact  methods.  To  estimate  with  the 
ophthalmoscope  which  lens  is.  required  to  give  an  eye 
emmetropic  vision,  three  very  important  facts  should  receive 
careful  attention  : 

1.  The  distance  between  the  surgeon's  and  patient's  eye. 

2.  The  surgeon's  and  patient's  accommodation. 

3.  The  surgeon's  own  refractive  error. 

First,  the  surgeon  should  have  his  eye  as  close  to  the 
patient's  eye  as  possible,  usually  at  13  mm.  ;  this  is  the 


I2O        REFRACTION  AND  HOW  TO  REFRACT. 

/anterior  principal  focus  of  the  eye,  and  is  the  distance  at 
'/which  the  patient  will  wear  his  glasses. 

Second,  as  already  explained,  the  observer's  and  patient's 
accommodation  should  be  in  repose.  The  most  difficult 
part  for  the  student  to  learn  is  to  relax  his  accommoda- 
tion. The  ambitious  student  strains  his  accommodation 
;  (ciliary  muscle)  in  his  haste,  and  with  the  result  that  he 
•'•  thinks  all  eyes  myopic  and  all  eye-grounds  as  affected 
with  "  retinitis." 

Third,  the  surgeon,  if  not  emmetropic,  must  wear  any 
necessary  correcting  lenses ;  otherwise,  the  lens  in  the 
ophthalmoscope  will  record  his  and  the  patient's  error 
together,  and  deductions  must  be  made  accordingly.  For 
instance,  if  the  surgeon  is  hyperopic  -f  2  S.,  and  does  not 
wear  his  glasses,  and  the  ophthalmoscope  records  the  fundus 
as  seen  clearly  with  -j-  5  S.,  this  would  mean  that  the  patient 
had  +3  S.  (2  of  the  5  S.  being  the  surgeon's  error);  or 
if  the  fundus  is  seen  without  any  lens  in  the  ophthalmoscope, 
then  the  patient's  error  would  be  — 2  S.  (the  surgeon's 
+  2  S.  from  o  leaving  — 2  S.)  ;  or  if  the  ophthalmoscope 
showed  — 2  S.,  then  the  patient's  error  would  be — 4  S. ; 
or  if  the  ophthalmoscope  registered  -\-2  S.,  then  the 
patient  would  be  emmetropic,  and  this  -j-2  S.  is  the  sur- 
geon's error. 

Rules. —  i.  When  the  surgeon  and  patient  are  both  hy- 
peropic or  both  myopic,  the  surgeon  must  subtract  his  cor- 
rection from  the  lens  which  shows  at  the  sight-hole  in  the 
)  J  ophthalmoscope. 

2.  When  the  surgeon's  eye  is  hyperopic  or  myopic,  and 

the  eye  of  the  patient  is  the  opposite,  he  must  add  his  cor- 

!  rection  to  the  lens  at  the  sight-hole  in  the  ophthalmoscope. 

With  the  foregoing  details  clearly  in  mind  and  carefully 
executed,  the  surgeon  selects  small  vessels  near  the  macula 


frt4A*X 


MYOPIA.  121 

for  his  observations.  If  it  .is  impossible  to  see  these  on 
account  of  the  small  pupil,  then  he  will  have  to  observe  the 
larger  vessels  at  the  disc  (nerve-head,  or  papilla). 

\Yhenever  the  vessels  in  the  macular  region  are  seen 
clearly  with  one  and  the  same  glass  in  the  ophthalmo- 
scope, the  refractive  error  can  be  approximated  as  one  of 
axial  ametropia,  and/every  three  diopters,  plus  or  minus,  or 
any  multiple  of  three  diopters,  represent  very  closely  one 
millimeter  of  lengthening  or  shortening  of  the  anteropos- 
terior  diameter  of  the  eye.\  For  example,  any  eye  that 
takes  a  plus  3  S.  to  make  it  emmetropic  is  just  I  mm.  too 
short  ;  any  eye  that  takes  a  minus  3  S.  to  make  it  emme- 
tropic is  about  i  mm.  too  long.  It  will  be  observed,  however, 
under  the  head  of  curvature  ametropia  (astigmatism),  that 
every  6  D.  cylinder  represents  about  I  mm.  in  length,  as 
measured  on  the  radius  of  curvature  of  the  cornea.  The 
following  table,  from  Nettleship,  gives  the  exact  equiva- 
lents in  millimeters  for  axial  ametropia  : 

H  .....     i  D.  =  o.3  mm.  M  .....     I  D.=o.3    mm. 


61).—  2       "  6  D.=  1.75  " 

9D.=  3        "  9D.=  2.6  " 

12  D.=4       "  I2D.=  3.5  " 

i8D.=  5 

Indirect  Method.  —  See  page  99  for  a  full  description  of 
this  method.    Slowly  withdrawing  the  objective  lens,  and  the 

[disc  remaining  unchanged  in  size,  signifies  emmetropia  ;  if 
the  disc  grows  uniformly  smaller,  it  means  H.,  and  if  it 
grows  uniformly  larger,  it  means  M.  (See  Fig.  142.)  This 
is  merely  a  method  of  diagnosis,  and  is  never  used  for 
definite  measurements. 
ii 


•   * 

CHAPTER  V. 

ASTIGMATISM,   OR   CURVATURE  AMETRO- 
PIA.— TESTS    FOR   ASTIGMATISM. 

Astigmatism  (from  the  Greek,  «,  priv. ;  a-l-fiia,  "a 
point"). — Optically,  astigmatism  may  be  defined  as  the  re- 
fractive condition  in  which  rays  of  light  from  a  point,  passing 
through  a  lens  or  series  of  lenses,  do  not  focus  at  a  point* 

In  ophthalmology  astigmatism  is  recognized  as  that  con- 
dition of  the  refractive  system  of  an  eye  in  which  rays  of 
light  are  not  refracted  equally  in  all  meridians,  and  the 
resulting  image  of  a  point  becomes  (an  oval,  a  line,  or  a 
circle.^  (See  Fig.  100.) 

Or  astigmatism  is  that  condition  of  an  eye  in  which  there 
are  two  principal  meridians,  of  greatest  and  least  ametropia, 
each  having  a  different  focus. 

In  the  standard  eye  the  cornea  is  represented  as  a  section 
of  a  sphere;  anatomically,  however,  the  cornea  is  generally 
found  to  be  an  ellipsoid  of  revolution,  with  its  shortest  radius 
of  curvature  (normally  7.8  mm.)  in  the  vertical  meridian. 

In  the  study  of  astigmatism  the  meridians  of  minimum 
and  maximum  refraction  alone  are  considered  ;  they  are 
spoken  of  as  the  principal  meridians,  and  are  at  right 
angles  to  each  other. 

With  very  few  exceptions  most  eyes  have  some  degree 
of  astigmatism.  The  standard  or  emmetropic  eye  is  an 

*In  an  article  published  by  Dr.  Swan  M.  Burnett,  in  "The  American 
Journal  of  Ophthalmology,"  for  December,  1903,  entitled  '•  Astigmia  or  Astig- 
matism," he  draws  attention  to  the  fact  that  astigmatism  is  an  erroneous  word, 
and  gives  the  true  origin  of  the  word  from  oriyfir]-r)^,  meaning  a  mathematic 
point,  whereas  "anyfia"  really  means  a  "blemish"  or  "brand."  He  there- 
fore urges  the  change  from  astigmatism  to  astigmia,  with  the  word  "astigmic'' 
as  the  adjective. 

122 


I23 

extremely  rare  condition,  and  plain  myopic  eyes  (long  eyes) 
i(.4t1iout  any  astigmatism  are  almost  as  rare  as  the  emme- 
tropic  condition  ;  and  while  plain  hyperopic  eyes  are  seen, 

--yet  statistics  show  that  fully  eighty  per  cent,  of  hyperopic 
eyes  have  astigmatism. 

..  Astigmatism  is  located  in  the  cornea  or  lens,  or  it  may 
be  a  condition  of  both  structures  in  one  and  the  same  eye. 
Astigmatism  of  the  lens  may  increase,  diminish,  or  neutral- 
ize the  corneal  astigmatism.  Astigmatism,  however,  is  more 

?  often  a  condition  of  the  cornea  than  of  the  lens.  ,    7  / 

Figure    100    shows    parallel    rays    of 
light  passing  through  an  astigmatic  lens 

in  which  the  vertical  meridian   has  the 

•  H' 

H 

-^  ^i— • — i — • 

V 


V 


FlG.   IOO. 


shortest  radius  of  curvature,  with  the  result  that  those  rays 
which  pass  through  the  vertical  meridian  V  V  come  to  a 
focus  before  those  in  the  horizontal  meridian  H  H',  which 
has  the  longest  radius. 

Intercepting  the  refracted  rays  at  I,  2,  3,  4,  5,  and  6,  the 
image  would  be  at  I  a  horizontal  oval,  at  2  a  horizontal 
line,  at  3  a  circle,  at  4  a  vertical  oval,  at  5  a  vertical  line, 
and  at  6  a  vertical  oval.  The  space  between  the  points  of 
foci  of  the  two  meridians  (2  and  5)  is  known  as  Sturm's 
interval.  The  importance  of  this  space  or  interval  is  that 


124        REFRACTION  AND  HOW  TO  REFRACT. 

it  represents  astigmatism.     Sturm's  interval  is  the  quantity 
which  must  be  found  in  correcting  astigmatism. 

Causes  of  Astigmatism. — Most  cases  of  astigmatism 
are  congenital,  and  some  can  be  traced  to  heredity.  Ac- 
quired astigmatism  may  result  from  conic  cornea,  cicatrices 
following  ulcers  or  wounds  of  the  cornea,  or  be  a  tempo- 
rary condition  from  pressure  of  a  chalazion  or  other 
growth  ;  and,  in  fact,  astigmatism  may  develop  from  any 
disease  or  injury  that  will  cause  a  lengthening  or  shortening 
/..  'or  inequality  in  one  or  more  of  the  meridians  of  the  cornea 
or  lens.  Swelling  of  the  different  sectors  of  the  lens  will 
cause  astigmatism.  The  visual  line  not  passing  through 
the  center  of  the  cornea  is  a  cause  of  astigmatism,  and 
astigmatism  is  the  usual  result  following  extraction  of  the 
\  xdens.  Tenotomy  of  one  or  more  of  the  extraocular  mus- 
cles will  often  change  the  corneal  curvature. 

Irregular  Lenticular  Astigmatism.-£This  is  a  normal 
y^  condition  of  all  clear  lenses.)  It  is  often  infinitesimal  in 
amount,  and  on  this  account  does  not  interfere  with  vision. 
It  is  caused  by  the  different  sectors  of  the  lens  or  by  the 
individual  lens-fibers  themselves  not  being  uniform  in  their 
refracting  power.  /In  this  form  of  astigmatism  a  light  does 
not  appear  to  have  a  distinct  edge,  but,  on  the  contrary, 
the  edge  has  radiations  passing  from  it,  giving  the  light  a 
stellate  qppparanrp.  ^  There  is  no  known  glass  that  vj -ilL  5 
correct  this  variety  of  astigmatism,  ^r*******  •**"  <*!*•  "**j* 

Physiologic  Astigmatism. — This  is  due  to  lid  pressure, 
or  temporarily  to  extreme  pulling  or  contraction  of  the  extra- 
ocular  muscles.  It  is  a  voluntary  astigmatism,  and  therefore 
not  constant.  It  is  not  a  condition  of  all  eyes.  The  writer 
has  demonstrated  with  the  retinoscope  and  ophthalmometer 
that  the  condition  can  be  produced  in  eyes  not  otherwise 
astigmatic.  Drawing  the  lids  together  in  the  act  of  squint- 


ASTIGMATISM.  125 

ing  or  frowning,  the  patient  can  press  the  cornea  from 
above  and  below,  and  give  the  horizontal  meridian  of  the 
cornea  a  longer  radius  of  curvature  and  the  vertical  meri- 
dian a  shorter  radius  ;  or  with  the  eye  looking  into  the 
telescope  of  the  ophthalmometer,  no  overlapping  of  the 
mires  is  noted,  but  in  some  instances  when  told  to  open  the 
eye  widely  and  "stare"  into  the  instrument,  as  much  as 
lj£  or  ^  of  a  diopter  of  astigmatism  may  be  recorded. 

This  "transient"  astigmatism  should  never  be  corrected 
with  a  glass. 

Subdivisions  of  Astigmatism. — In  addition  to  the 
astigmatisms  just  described,  curvature  ametropia  has  been 
further  considered  as  : 

1.  Irregular.  6.  Astigmatism  against  the 

2.  Regular.  rule. 

3.  Symmetric.  7.   Homonymous. 

4.  Asymmetric.  8.    Heteronymous. 

5.  Astigmatism    with    the        9.   Homologous. 

rule.  10.   Heterologous. 

1.  Irregular  Astigmatism. — This  is  usually  located  in 
^the   cornea,  and   is   due  primarily  to    some   breach  in  the 

continuity  of  one  or  more  of  its  meridians  ;  for  example, 
the  vertical  meridian  may  appear  regular,  but  the  hori- 
zontal meridian  is  not  a  uniform  curve,  but  is  irregular  at 
some  point  or  points.  Such  meridians  can  not  produce 
clear  retinal  images,  but,  on  the  contrary,  the  resulting 
retinal  image  is  hazy  or  irregular. 

2.  Regular  Astigmatism. — In  this  variety  the  cornea 
and  lens  are  regular  in  their  curvatures,  from  the  maximum 
to  the  minimum  radius,  and  the  retinal  image  can  be  made 
clear  with  correcting  glasses. 

Before  entering  upon  the  study  of  the  various  forms  of 
regular  astigmatism,  the  student's  attention  is  called  to  two 


126        REFRACTION  AND  HOW  TO  REFRACT. 

important  facts  :  (a)  That,  as  a  rule,  the  shortest  radius  of 
curvature  of  the  cornea  is  in  the  vertical  meridian — that  is 
to  say,  the  vertical  meridian  has  a  stronger  refracting  power 
than  the  horizontal. 

(ti)  The  student  should  bear  in  mind  that  in  the  measure- 
ment of  curvature  ametropia  each  millimeter  of  lengthening 
or  shortening  of  the  radius  of  curvature  is  equivalent  to  a 
6  D.  cylinder.  For  instance,  an  eye  which  requires  a  -\-6  D. 
cylinder  axis  90  degrees  has  the  horizontal  radius  of  curva- 
ture about  one  millimeter  longer  than  the  vertical  radius ; 
or  an  eye  that  requires  a  — 6  D.  cylinder  axis  180  degrees 
has  its  vertical  radius  of  curvature  about  one  millimeter 
shorter  than  the  horizontal.  In  axial  ametropia,  however, 
it  was  shown  that  every  three  diopter  sphere  represented 
about  one  millimeter  in  length,  as  measured  on  the  axis. 

Varieties  of   Regular  Astigmatism. — There  are  five 
different  forms  of  regular  astigmatism  : 
(a)  Simple  hyperopic.  (r)    Compound  hyperopic. 

(fi)  Simple  myopic.  (<•/)  Compound  myopic. 

(e)  Mixed  astigmatism. 

(a]  Simple  Hyperopic  Astigmatism. — Abbreviated  As. 
H.,  or  H.  As.,  or  Ah.  /About  5^  per  cent,  of  eyes  have 

this  form  of  refraction.)  This  is 
a  condition  where  one  meridian 
of  the  eye  is  emmetropic,  and 
the  meridian  at  right  angles  to 
it  is  hyperopic  (see  Fig.  101); 
the  vertical  meridian  focuses 
parallel  rays  on  the  retina,  and 
the  horizontal  meridian  would 

focus  back  of  it.  The  retinal  image  of  a  point  is  a  line, 
usually  horizontal.  (See  2,  in  Fig.  100.)  The  correcting 
lens  is  a  plus  cylinder  with  its  axis  usually  at  90  degrees, 


ASTIGMATISM. 


127 


FIG.  102. 


or  within  45  degrees  of  90  degrees.  Example,  -f-  2.00 
cylinder  axis  90  degrees. 

(fr)  Simple  Myopic  Astigmatism. — Abbreviated  As.  M., 

or  M.  As.,  or  Am.    This  is  not  a  common  condition.    About 

>  I  y2  per  cent,    of  all  eyes  have   this  form  of  astigmatism. 

/This  is  a  condition  where  one  meridian  of  the  eye  is  emme- 

1  tropic,  and  the  meridian  at  right  angles  to  it  is  myopic  (see 

Fig.  102);  the  horizontal  meridian 

focuses  parallel  rays  on  the  retina, 

and  the  vertical  meridian  focuses 

parallel  rays  in  front  of  the  retina 

(in  the  vitreous),  with  the   result 

that    they   cross   before    reaching 

the  retina.    /The  retinal  image  of 

a  point  is  a  line,  usually  vertical. 

(See  Fig.  100.)  The  correcting  lens  is  a  minus  cylinder  with 
its  axis  at  180  degrees,  or  within  45  degrees  of  180  degrees. 
Example,  — 2.50  cylinder  axis  180  degrees. 

(r)  Compound  Hyperopic  Astigmatism. — Abbreviated 
H.  As.  Co.,  or  Comp.    Has.,  or  H-|-Ah  (hyperopia   com- 
bined with  astigmatism  hyperopic).      This  condition  repre- 
~>sents  nearly  forty-four  per  cent,  of  all  eyes ;  it  is  the  most 

common   of  all   forms  of  re- 
fraction. 

The    retinal     image    of    a 
point  is  an  oval  ;   never  a  line 
and  never  a    circle.     (See   I/ 
in  Fig.  100.) 

F,G.  I03,  The  correcting  lenses  are  a 

plus  sphere  and  a  plus  cylin- 
+  3.00  cylinder  axis   90  de- 


der. 


Example,  -}-2.oo  S. 
grees.      Compound  hyperopic  astigmatism  is  a  combination 
of  axial  ametropia  (short  eye)  and  simple  hyperopic  astig- 


128        REFRACTION  AND  HOW  TO  REFRACT. 

matism  (curvature  ametropia).  In  this  form  of  astigmatism 
both  meridians  have  their  foci  back  of  the  retina — one 
further  back  than  the  other.  The  retina  intercepts  the  rays 
before  they  can  focus.  Figure  103  shows  this  condition. 
/  Usually  the  vertical  meridian  focuses  nearer  the  retina  than 
the  horizontal. 

(d)  Compound  Myopic  Astigmatism. — Abbreviated  M. 
As.  Co.,  or  Comp.  Mas.,  or  M.  H-Am.  (myopia  combined 
with  astigmatism  myopic).  This  is  by  far  the  most  com- 
mon condition  of  all  myopic  eyes,  and  represents  about 
eight  per  cent,  of  all  eyes. 

The  retinal  image  of  a  point  is  always  an  oval ;  never  a 
line  or  a  circle.  (See  6,  in  Fig. 
100.) 

The  correcting  lenses  are  a 
minus  sphere  and  a  minus  cylin- 
der. Example,  — i  sph.  O — - 
cylinder  axis  180  degrees.  A 
combination  of  axial  ametropia 
FIG.  104.  (long  eye)  and  simple  myopic 

astigmatism. 

Figure  104  shows  that  parallel  rays  have  t\vo  points  of 
foci  in  front  of  the  retina — one  further  front  than  the 
other. 

(c)  Mixed  Astigmatism. — This  form  of  refraction  is 
found  in  about  6^  per  cent,  of  all  eyes,  and  is  abbreviated 
in  th/ee  different  ways  : 

I/  Ah-fAm.  (astigmatism  hyperopic  with  astigmatism 
myopic). 

2.   H-f-Am.  (hyperopia  with  astigmatism  myopic). 
/  3.   M-j-Ah.  (myopia  with  astigmatism  hyperopic). 
/  The  retinal  image  of  a  point  is  an  oval  or  a  circle ;  never 
line.     (See  3  and  4  in  Fig.  100.) 

j^-i/'-'A— *~^L*{      ^*- 


. 


ASTIGMATISM. 


129 


The  correcting  lenses  are  one  of  three  combinations,  and 
spoken  of  as  crossed  cylinders.      Examples  : 

1.  +1.00  cyl.  axis  90  degrees  ^  — 2.00  cyl.  axis  180  degrees. 

2.  -j-i  S.  O  — 3  c)'l-  ax's  I8°  degrees  (cylinder  always^ stronger   than  the 
sphere). 

3.  — 2  S.  ^  -f~3  cyl.  ax'5  9°   degrees  (cylinder  always  stronger  than  the 
sphere) . 

'    The  condition  of  mixed   astigmatism   is  one  of  simple 

7hyperopic   astigmatism,  with  simple    myopic    astigmatism  : 

one  meridian  focuses  parallel  rays  in  front  of  the  retina  and 

the  other  meridian  (at  right  angles)  focuses  parallel   rays 


FIG.  105. 


FIG.  106. 


back  of  the  retina.  Figures  105  and  106  show  this  arrange- 
ment. 

The  remaining  subdivisions  of  astigmatism  are  merely 
classifications  of  the  different  forms  already  described,  and 
arise  from  a  study  of  the  axis  of  shortest  radius  of  curva- 
ture. 

3.  Symmetric  Astigmatism.  —  \Yhen  the  combined 
values,  in  degrees,  of  the  meridians  of  shortest  or  longest 
radii  of  curvature  in  both  eyes  equal  180  degrees  (no  more 
and  no  less),  then  the  astigmatism  in  the  two  eyes  is  spoken 
of  as  symmetric.  For  example,  if  the  cylinder  in  the  right 
eye  is  at  axis  75  degrees,  and  in  the  left  eye  at  105  de- 
grees ;  75  degrees  and  105  degrees  added  together  will 


130 


REFRACTION    AND    HOW    TO    REFRACT. 


make  180  degrees.  (See  Fig.  107.)  Or  if  each  eye  takes 
a  cylinder  axis  at  90  degrees,  they  are  also  symmetric,  90 
degrees  and  90  degrees  making  180  degrees.  If  both  eyes 
have  axes  1 80  degrees,  they  are  symmetric  also,  one 
meridian  being  considered  as  zero  (o). 

4.  Asymmetric  astigmatism    is  the  reverse    of  sym 

H.JL/'  metric,  and  is,  therefore,  the  condition  in  which  the  com- 
bined values,  in  degrees,  of  the  cylinder  axes  do  not  make 
1 80  degrees.  For  instance,  if  the  right  eye  has  a  cylinder 
aj^fxis  75  degrees  and  the  left  at  120  degrees,  these 

'  '    /  t/l 

.". 


93' 


13 


180     0 


135 


FIG.    107.  —  Illustrating   Symmetric 
Astigmatism. 


135 


75    90 


120 


FIG.  108. — Illustrating  Asymmetric 
Astigmatism. 


added  together  would  not  make  1 80  degrees,  but  more  than 
1 80  degrees.  (See  Fig.  108.)  Or,  if  the  astigmatism  in 
the  right  eye  was  at  35  degrees,  and  the  left  at  90  degrees, 
these  added  together  would  not  make  180  degrees. 

Symmetric  astigmatism  generally  accompanies  a  regular 
physiognomy,  the  center  of  each  pupil  being  at  an  equal 
distance  from  the  median  line  of  the  face.  Asymmetric 
astigmatism  usually  accompanies  an  asymmetric  physiog- 
nomy, the  center  of  one  pupil  being  further  from  the 
median  line  of  the  face  than  the  other. 


ASTIGMATISM. 


13  1 


Muscular  insufficiency,  hereafter  to  be  described,  is  much 
more  common,  and,  in  fact,  should  be  looked  for  or  antici- 
pated in  cases  of  asymmetric  astigmatism.  ^^*^  *flf 

5  and  6.  Astigmatism  with  the  Rule  and  Astigmatism 
Against  the  Rule. — Astigmatism  with  the  rule  and  astig- 
matism against  the  rule  refer  to  the  condition  already 
)  described  as  that  in  which  the  vertical  meridian  of  the  eye, 
as  a  general  rule,  has  the  shortest  radius  of  curvature. 

Statistic  tables  on  astigmatism  show  that  most  eyes 
a  plus  cylinder  at  axis  90  degrees,  or  within  45 


135, 


45 


FIG.  109. — Illustrating  Astigmatism 
with  the  Rule. 


135° 


FIG.  no. — Illustrating  Astigmatism 
aguinst  the  Rule. 


degrees  (inclusive)  either  side  of  90  degrees  (see  Fig. 
109);  or  a  minus  cylinder  at  axis  180  degrees,  or  within 
45  degrees  (inclusive)  either  side  of  180  degrees.  For 
example,  if  an  eye  requires  a  plus  cylinder  at  45  degrees, 
or  at  any  axis  from  45  degrees  up  to  135  degrees  (inclu- 
sive), taking  axis  90  as  the  median  line,  then  the  astig- 
--jnatism  is  u'ith  the  rule.  But  if  an  eye  should  require  a 
plus  cylinder  within  45  degrees  either  side  of  180  degrees, 
then  the  condition  is  one  of  astigmatism  against  the  rn/e. 
(See  Fig.  1 10.)  A  plus  or  minus  cylinder  at  45  degrees 


132        REFRACTION  AND  HOW  TO  REFRACT. 

or    135    degrees    is    recognized    as    astigmatism    with    the 

,     ™lC'         &~-*9     >* *»       *V~,— -    **— ^ 

f   7.  Homonymous  astigmatism  is  the  condition  in  which 
/  the  cylinder  axis  in  each  eye  is  the  same.) 

t        ^  '  *  £  _ —  f-C»^«_  ««-  «-*-»  1 

8.  Heteronymous  astigmatism  is  the  condition  in  which 
the  astigmatism  in  one  eye  is  with  the  rule,  and  in  the  other 
eye  against  the  rule.  For  example  :  *• 

O.  D.  4-2  cyl.  axis  90  degrees,  and  O.  S.  4  2.00  cyl.  axis  180  degrees. 

?"       Q.  Homologous  astigmatism  is  symmetric  astigmatism 
**with  the  rule — /.  e. : 

O.  D.  4  l.oo  cyl.  axis  60  degrees,  O.  S.  -j-l-oo  cyl.  axis  120  degrees. 

10.  Heterologous  astigmatism  is  symmetric  astigma- 
tism against  the  rule — /.  e.  : 

O.  D.  4-I-OO  cyl.  axis  15  degrees,  O.  S.  -\-i.O3  cyl.  axis  165  degrees. 

Meridians  of  the  Eye. — The  various  axes  or  meridians 
of  the  eye  are  indicated  by  degree  markings  on  the  peri- 
phery of  the  trial-frame,  and  by  corresponding  imaginary 
lines  drawn  around  the  eyeball  from  the  anterior  pole  or 
apex  of  the  cornea  to  the  posterior  pole. 

Either  eye  (right  or  left)  is  exactly  like  its  fellcnv,  and  is 
numbered  by  starting  from  zero  (o)  on  the  left-hand  side  of 
the  horizontal  meridian  and  counting  downward  to  the  right- 
hand  side  until  this  same  line  is  again  reached.  This  makes 
half  a  circle  (hemisphere)  of  180  degrees.  (See  Fig.  109.) 
As  the  degrees  in  this  half-circle  are  all  carried  across  the 
eye,  they  maintain  their  individual  numbering,  so  that  axes 
5,  10,  15,  etc.,  are  the  same  whether  above  or  below  the 
horizontal  meridian.  Hence  there  is  no  reason  for  having 
a  complete  circle  of  360  degrees.  Some  trial -frames  have 
the  upper,  while  others  have  the  lower,  half  numbered ; 
this  makes  no  difference  in  the  exact  numbering ;  in  one  in- 
stance the  count  is  made  from  left  to  right,  and  in  the  other 


ASTIGMATISM.  133 

the  count  is  made  from  right  to  left.  The  foreign  trial-frame, 
as  represented  on  page  48,  may  be  confusing  if  not  studied. 

Symptoms  of  Astigmatism. — More  aggravated  symp- 
toms of  accommodative  asthenopia  are  apt  to  be  detailed 
by  the  patient,  but  there  are,  in  truth,  no  definite  symptoms 
whereby  the  presence  of  astigmatism  can  be  positively,  dif- 
ferentiated from  axial  ametropia.  The  diagnosis  of  astig- 
matism by  the  physiognomy  is  confirmed  only  because 
most  eyes  are  astigmatic  ;  the  simple  hyperopic  eye  squints 
the  eyelids  together  just  the  same  as  the  eye  that  is  astig- 
matic, so  that  the  writer  would  not  diagnose  astigmatism 
by  the  patient's  individual  history  of  his  eyes. 

How  to  Diagnose  Astigmatism. — This  is  one  of  the 
very  early  questions  of  the  beginner  in  ophthalmology. 
Astigmatism  being  the  prominent  factor  in  almost  all  refrac- 
tive work,  the  writer  feels  justified  in  giving  this  part  of 
refraction  extensive  explanation.  Of  the  various  methods 
of  diagnosing  astigmatism  the  writer  would  mention  the 
following  : 

1.  Corneal  reflex.  IO.  Chromo-aberration   or    cobalt-blue 

2.  Confusion  letters.  glass  test. 

3.  Placido's  disc.  II.  Thomson's  ametrometer. 

4.  Stenopeic  slit.  12.  The  ophthalmometer. 

5.  Astigmatic  chart.  13.    Direct  ophthalmoscopy 

6.  The  pointed  line  test.  14.   Indirect  ophthalmoscopy. 

7.  Perforated  chart  or  disc.  15-    Cylindric  lenses. 

8.  Fray's  letters.  1 6.  Retinoscope. 

9.  Schemer's  Test. 

I.  The  Corneal  Reflex  Test. — The  cornea  and  under- 
lying aqueous  representing  a  spheric  mirror,  naturally  fur- 
nish a  small  image  of  surrounding  objects.  If  the  cornea 
is  astigmatic,  the  catoptric  image  must  be  correspondingly 
distorted.  To  make  the  examination,  the  patient  stands 
facing  a  window,  and  the  surgeon  at  one  side  observes  the 


134        REFRACTION  AND  HOW  TO  REFRACT. 

image  of  the  window-panes  in  the  conical  mirror  ;  these 
will  be  broadened  or  lengthened,  or  they  may  appear  in- 
clined to  one  side,  according  to  the  axis  and  character  of 
the  astigmatism.  This  test  is  not  commonly  used,  is  often 
overlooked ;  in  fact,  unless  the  astigmatism  is  of  consid- 
erable degree,  is  not  a  valuable  test. 

2.  Confusion  Letters. — Letters   on   the   card  which  is 
used  for  testing  distant  vision  are  arranged  in  such  order 
that  those  which  have  a   resemblance  are  placed  next  to 
each  other.     (Fig.  74.)     For  example,  X  and  K,  Z  and  E, 
O    and   D,   C  and   G,  P  and   F,  S    and   B,  V  and   Y,  H 
and  N,  A  and   R,  etc.     The  patient,  in  deciphering  these 
letters  in  the  line  corresponding  to  his  best  vision,  often 
miscalls  them,  and   can   not  tell   an  X  from  a  K,  or  a  Z 
from  an  E,  etc.     These  letters  are,  therefore,  spoken  of  as 
confusion  letters.  (This  is  a  very  good  general  test,  but  is 
not  infallible,  as  a  patient  with  opacities  in  the  media  will 
make  similar  mistakes./ 

3.  Placido's  Disc  or  Keratometer  (see  Fig.  1 1 1). — To  a 
wooden  handle  is  secured  a  round  piece  of  thin  sheet-iron 
eight  inches  in  diameter,  and  at  its  center  is  a  small,  round 
5  mm.  opening.     On  one  side  the  disc  is  painted  in  alternate 
concentric  circles  or  bands  in  black  and  white ;  these  circles 
are  not  equidistant,  the  radii  of  the  several  circles  being  cal- 
culated according  to  the  law  of  tangents,  so  that  when  re- 
flected on  a  cornea  of  spheric  curvature  they  appear  equi- 
distant in  the  image.    On  the  reverse  side  is  placed  a  slot  to 
hold  a  convex  lens  for  magnifying  purposes.      To  use  this 
disc,  the  patient  is  placed  with  his  back  to  a  strong  light  from 
a  window,  or  an  artificial  light  may  be  placed  over  his  head. 
The  surgeon  holds  the  disc  with  the  sight-hole  close  in  front 
of  his  own  eye,  and  with  the  light  illuminating  the  disc,  the 
patient  is  instructed  to  look  into  the  perforation.     The  sur- 


ASTIGMATISM. 


135 


geon  then  approaches  the  eye  until  the  corneal  image  of 
the  outer  edge  of  the  instrument  corresponds  to  the  outer 
edge  of  the  patient's  cornea.  When  this  distance  is 
reached,  a  convex  2,  3,  or  4  D.  sphere  may  be  placed  in 
the  slot  of  the  disc  so  as  to  magnify  the  corneal  image. 
(If  the  cornea  is  not  astigmatic,  then  the  black  and  white 

circles  will  appear  uni- 
form throughout ;  but  if 
there  is  astigmatism,  the 
circles  will  appear  more 
or  less  oval.  If  irregu- 
lar astigmatism  or  conic 
cornea  is  present,  the  cir- 
cles will  appear  broken 
or  distorted  in  certain 
parts.  This  test  has  be- 
come  almost  obsol 


FIG.  in. 


FIG.  112. 


4.  Stenopeic  Slit  (see  Fig.  1 12). — This  is  a  round  metal 
disc  of  the  size  of  the  trial-lens,  and  contains  a  central  slit 
or  opening  about  25  mm.  long  and  I  or  2  mm.  wide.  The 
stenopeic  slits  sold  in  the  shops  have  various  breadths  of 
openings,  from  ^  to  2  mm.  ;  that  with  the  I  mm.  opening 


136        REFRACTION  AND  HOW  TO  REFRACT. 

is  the  one  recommended.  The  purpose  of  the  slit  is  to 
cut  off  or  exclude  all  rays  of  light  at  right  angles  to  its 
position  in  front  of  the  eye.  When  placed  at  axis  90,  all 
rays  in  the  horizontal  meridian  are  excluded  ;  when  placed 
at  axis  1 80,  all  rays  in  the  vertical  meridian  are  cut  off,  etc. 
jTo  use  the  stenopeic  slit,  place  it  in  the  trial-frame  in  front 
I  of  the  eye  to  be  examined,  the  fellow-eye  being  covered. 
The  patient  is  instructed  to  read  the  letters  on  the  distant 
test-card,  and  as  he  does  so,  the  slit  is  slowly  turned 
through  the  different  meridians.  If  the  vision  remains  the 
same,  no  matter  through  which  meridian  the  patient  reads, 
astigmatism  may  be  absent ;  but  if  the  patient  selects  one 
meridian  in  which  he  sees  best,  and  another  meridian  at 
right  angles  in  which  he  does  not  see  so  well,  astigmatism  is 
usually  present.  For  instance,  if  the  slit  is  at  axis  75  degrees 
and  the  patient  reads  ^,  and  at  axis  165  he  reads  ~^,  then 
he  is  astigmatic  in  the  165  meridian.  The  amount  of  the 
astigmatism  can  be  calculated  by  placing  spheric  lenses 
back  of  the  slit  and  finding  the  difference  in  strength  of 
the  spheres  which  bring  the  vision  up  to  the  normal.  For 
example,  when  the  slit  is  at  axis  75  and  the  patient  reads 
f^,  if  a  -f  1.50  S.  is  used,  and  the  vision  becomes  yj,  then 
1.50  corrects  axis  75.  Turning  the  slit  to  axis  165,  and 
proceeding  in  the  same  way,  if  -f-  2. 50  S.  brings  the  vision 
from  ^  to  ^},  then  +2.50  corrects  axis  165,  the  differ- 
ence between  the  +1.50  and  +2.50  being  i  D.,  and  the 
formula  would  be  +1.50  sph.  ^  -f  i.oo  cyl.  axis  75 
degrees.  This  test  is  not  often  used,  and  when  resorted 
to,  the  eyes  should  be  under  the  influence  of  a  cyclo- 
plegic.  This  test  is  of  special  service  in  some  cases  of 
mixed  astigmatism,  irregular  astigmatism,  presbyopia,  and 
aphakia. 


ASTIGMATISM. 


137 


5.  Astigmatic  Chart. — There  is  an   infinite  variety  of 
these    cards   (see  Fig.    113),  and    the    student   is   puzzled 


FIG.  113. — Astigmatic  Charts  of  Dr.  John  Green. 

which  one  to  select.      Ordinarily,  the    "  clock-dial "  will 
answer   every   purpose.       (Fig-    H4-)      This    is    a    white 


138 


REFRACTION  AND  HOW  TO  REFRACT. 


card  *  with  peripheral  Roman  characters  corresponding  to 
the  characters  on  the  clock-face,  hence  its  name.  From 
these  figures  a  series  of  three  parallel  and  uniformly  black 
lines,  with  interspaces  of  the  same  width  as  the  lines,  cross 
from  XII  to  VI,  III  to  IX,  IIII  to  X,  V  to  XI,  VII  to  I, 
and  VIII  to  II.  This  chart  should  be  so  calculated  that 


FIG.  114. 

the  lines  and  interspaces  will  form  an  angle  of  5  minutes  in 
width  consistent  with  the  distance  at  which  the  test  is  to  be 
/made  :  if  at  six  meters,  8.7  mm.  ;  if  at  four  meters,  5.7  mm. 
In  most  charts  the  lines  subtend  an  angle  much  greater 
than  5  minutes  for  the  distance  at  which  they  are  used,  and 
in  this  way  the  true  delicacy  of  the  test  for  small  errors  or 


*  A  black  card  with  white  lines  is  also  used.      (See  Fig.  115.) 


ASTIGMATISM. 


139 


amounts  of  astigmatism  is  sacrificed.  The  purpose  of  the 
chart  is  to  detect,  by  the  patient's  answer,  whether  astigma- 
tism is  present,  and,  if  so,  in  which  meridian. 

The  chart,  illuminated  by  reflection  from  a  steady  artificial 
light,  is  placed  on  a  horizontal  line  perpendicular  to  the 
patient's  eyes  ;  it  should  never  be  hung  at  an  angle,  and 
must  always  be  perfectly  flat.  Each  eye  is  to  be  tested 
separately.  Looking  at  such  a  chart,  if  all  the  lines  appear 


FIG.  115. 


equally  black,  astigmatism  of  any  considerable  degree  or 
amount  may  often  be  excluded ;  but  if  the  patient  selects 
one  series  of  lines  as  darker  than  others,  then  the  presence 
of  astigmatism  may  be  diagnosed.  'If  the  astigmatism  is 
of  a  very  high  degree,  the  patient  may  see  the  three  lines 
as  one  solid  black  line  without  interspaces.J 

RULE  i. — The  meridian  of  the.  eye  which  corresponds  to 
the  dark  lines  selected  is  the  meridian  of  astigmatism. 

Example. — If  the  horizontal  lines  (from  III  to  IX)  appear 


I4O        REFRACTION  AND  HOW  TO  REFRACT. 

darker  than  all  the  others,  then  it  is  the  horizontal  meridian 
(o  or  1 80  degrees)  of  the  eye  which  is  astigmatic.  Or  if 
the  lines  from  VI  to  XII  are  darkest,  then  the  vertical  mer- 
idian of  the  eye  is  astigmatic.  In  other  words,  the  series 
of  darkest  lines  indicates  the  meridian  of  greatest  ametropia. 

RULE  2. — The  axis  of  the  cylinder  in  the  prescription 
will  be  opposite  to  the  meridian  of  the  dark  lines. 

Example. — A  patient  who  requires  a  plus  cylinder  at  axis 
90  degrees  sees  the  horizontal  lines  (from  III  to  IX)  as  very 
dark,  and  the  lines  from  VI  to  XII  not  so  dark,  and  the  axis 
of  the  cylinder  in  the  prescription  will  be  opposite  to  180 
degrees — i.  e.,  at  90  degrees. 

^According  to  the  definition  of  "astigmatism  with  the 
rule  "  and  "  astigmatism  against  the  rule,"  it  follows  that, 
with  few  exceptions,  those  patients  who  select  a  series 
of  lines  at  180  degrees,  or  within  45  degrees  either  side  of 
1 80  degrees,  as  darker  than  other  lines,  have  hyperopic 
astigmatism,  whereas  those  who  select  a  series  of  lines  at  90 
degrees,  or  within  45  degrees  either  side  of  90  degrees, 
have  myopic  astigmatism.  J 

According  to  the  definition  of  symmetric  astigmatism,  a 
patient's  right  eye  selecting  the  lines  at  90  or  180  as 
darker  than  those  at  right  angles,  will  select  the  same  series 
of  dark  lines  in  the  left  eye.  If  the  series  of  dark  lines  with 
the  right  eye  is  from  II  to  VIII,  then  the  left  eye  selects 
the  dark  lines  from  X  to  IV,  etc. 

The  clock-dial  is  the  form  of  chart  in  common  use, 
and  as  a  test  for  astigmatism .  is  not  without  considerable 
merit. 

When  the  astigmatism  is  of  small  amount,  it  may  not  be 
recognized  by  means  of  the  clock-dial  until  after  the  spheric 
correction  has  been  placed  before  the  eye  or  after  a  cyclo- 
plegic  has  been  instilled. 


ASTIGMATISM.  14! 

6.  The  writer's  pointed  line  test,  as  shown  in  figure  1 16, 
is  a  series  of  one-minute  black  squares,  in  three  parallel 
lines  at  right  angles  to  each  other,  on  a  cream-colored  card  ; 
the  squares  and  adjoining  spaces  making  a  five-minute 
angle  for  six  meters.  By  means  of  a  clockwork  and  bat- 
tery, this  dial  may  be  revolved  by  pressing  a  button.  The 
principle  of  the  test  is  the  same  as  the  perforated  disc. 


FIG.  1 1 6. 

7.  The  Perforated  Disc  (Fig.  117). — This  is  a  modifica- 
tion of  the  astigmatic  chart.  A  piece  of  white  cardboard 
or  metal,  about  ten  inches  square,  has  small,  round  perfora- 
tions made  in  it  of  certain  definite  size.  ^Each  perforation  is 
separated  from  its  neighbor  by  the  distance  of  its  diameter.^) 
These  openings  are  arranged  in  series  of  one,  two,  or  three 
parallel  lines,  exactly  as  in  the  pointed  line  test.  This 
chart  or  disc  is  hung  on  the  window-pane,  or  an  illumina- 
tion is  placed  behind  it.  The  patient,  looking  at  the  disc, 


142        REFRACTION  AND  HOW  TO  REFRACT. 

signifies  which  scries  of  perforations  appear  to  coalesce  and 
form  lines.     This  test  is  not  commonly  known  or  used.     It 


FIG.  117. 

might  be  a  valuable  test  if  there  was  any  convenient  way 
of  uniformly  illuminating  it  from  behind. 

8.  Fray's    Letters    (Fig.    118).- 

^zsJT   ^^    a§?^     These  letters  are  of  the  Old  English 
~  ^rz       5    5        55^^^ 
=~      "^?     5^8        type,  and  composed  of  strokes  which 

Ilium  //«i  •/////  run  ^n  different  meridians.  The  pa- 
flliiill  %/r  J/  ft  ^ent>  looking  at  these  letters,  selects 
that  letter  which  appears  darker  than 
all  the  rest.  The  direction  of  the  lines 
jn  the  letter  selected  corresponds  to  the 
meridian  of  greatest  ametropia.  (This 

^" 

test  is  very  confusing  to  the  patient, 
FIG.  118.  wno  sees  first  one  letter  and  then  an- 

other as  darker  than  its  fellows.^/ 
Schemer's  Test.  —  This  is  an  old  test  for  ametropia, 


\\ 


\A\V 


ASTIGMATISM. 


143 


good  in  theory,  but  really  not  sufficiently  accurate  for  prac- 
tical purposes.  It  is  explained  for  the  student's  information, 
and  not  with  the  idea  that  he  will  ever  take  time  to  use  it. 
The  test  is  made  with  a  small  piece  of  metal  (Fig.  119) 
the  size  of  the  trial-lens,  which  contains  two  pin-point  round 
openings  at  its  center,  separated  by  an  interval  of  two  or 
three  millimeters.  (One  of  these  openings  is  covered  with  a 
red  glass,  as  suggested  by  Dr.  Wm.  Thomson.)  This  disc 
is  placed  close  to  the  eye,  so  that  light  may  pass  through 
both  openings  into  the  eye  at  one  and  the  same  time.  The 
eye,  if  not  presbyopic,  should  be  under  the  influence  of  a 


Fie.  119. 


FIG.  120. 


cycloplegic.  The  eye  looks  at  a  distant  point  of  light  The 
principle  of  the  test  depends  upon  which  part  of  the  retina 
is  stimulated  by  the  rays  entering  the  eye  through  these 
openings,  all  other  rays  being  excluded.  The  student  must 
remember  that  rays  which  fall  upon  the  temporal  side  of  the 
retina  are  referred  to  the  nasal  side  ;  those  which  fall  upon 
the  nasal  side  of  the  retina  are  referred  to  the  temporal  side  ; 
those  which  fall  upon  the  lower  portion  of  the  retina  appear 
to  come  from  above  ;  and  those  which  fall  upon  the  upper 
portion  of  the  retina  appear  to  come  from  below. 

Diagnosis  of  Hyperopia  (  Fig.  1  20).  —  The  disc  is  placed 
with  the  red  glass  (R)  above.      The  patient  then  sees  a  red 


144 


REFRACTION  AND  HOW  TO  REFRACT. 


FlG.  121. 


and  a  white  light  (W).  The  red  appears  below  the  white. 
Gradually  revolving  the  disc,  the  two  lights  move,  and  keep 
the  relative  positions  and  distance  apart.  The  greater  the 
distance  between  the  two  lights,  the  higher  the  refraction 
or  amount  of  the  hyperopia.  That  plus  sphere  placed  in 
front  of  the  disc  which  unites  the  two  flames  into  one  (pink) 
flame  is  the  approximate  amount  of  the  hyperopia. 

Diagnosis    of    Myopia    (Fig.    121).  —  Placing    the  disc 

before  the  eye  as  before,  with 
the  red  glass  (R)  above,  the 
patient  sees  the  red  flame 
above  the  white  (W).  Gradu- 
ally revolving  the  disc,  these 
two  lights  keep  their  rela- 
tive positions  and  distance. 
That  minus  sphere  placed 
before  the  disc  which  makes 

the  two  lights  appear  as  one  (pink)  light  is  the  approximate 
amount  of  the  myopia. 

Diagnosis  of  Emmetropia.  —  This  condition  would  give 
but  one  light  (pink  in  color),  and  unchanged  by  rotating 
the  disc. 

Diagnosis  of  Simple  Hyperopic  Astigmatism.  —  One 
meridian  appears  the  same  as  in  emmetropia,  and  the  meri- 
dian opposite  to  the  emmetropic  meridian  would  show  a 
separation  of  the  two  lights,  as  in  hyperopia.  The  plus  cyl- 
inder placed  before  the  disc  which  unites  the  two  lights  in  the 
ametropic  meridian  represents  the  amount  of  the  astigma- 
tism. 

Diagnosis  of  Simple  Myopic  Astigmatism.  —  The 
lights  are  red  and  white  in  one  meridian,  as  in  simple  hyper- 
opic  astigmatism,  but  the  red  light  is  seen  in  the  direction 
of  the  red  glass,  and  when  the  disc  is  rotated  to  the  oppo- 


ASTIGMATISM.  145 

site  meridian,  only  one  light  appears,  and  of  a  pink  color. 
The  amount  of  the  astigmatism  is  represented  by  the 
strength  of  minus  cylinder  which  brings  the  two  lights 
together  in  the  ametropic  meridian. 

Diagnosis  of  Compound  Hyperopic  Astigmatism. — All 
meridians  show  two  lights,  the  red  light  being  in  the  direc- 
tion of  the  clear  opening  in  the  disc,  but  one  meridian  will 
show  a  greater  separation  of  the  lights  than  in  the  meridian 
at  right  angles.  To  find  the  correction  and  the  amount  of 
the  astigmatism,  proceed  as  in  simple  hyperopia,  correcting 
each  meridian  separately  with  a  sphere. 


FIG.  122. 


FIG.  123. 


Diagnosis  of  Compound  Myopic  Astigmatism. — This 
is  the  same  as  in  compound  hyperopic  astigmatism,  with  a 
reversal  of  the  position  of  the  lights,  and  the  amount  of  the 
ametropia  is  obtained  with  minus  spheres. 

Diagnosis  of  Mixed  Astigmatism. — One  meridian 
appears  as  in  simple  hyperopic  astigmatism,  and  the  meri- 
dian opposite  to  it  appears  as  in  simple  myopic  astigma- 
tism. The  amount  of  the  astigmatism  is  calculated  as  in 
these  two  conditions. 

10.  Chromo-aberration  Test. — This  is  also  known 
as  the  cobalt-blue  glass  test.  Cobalt  is  a  mineral,  and  is 
13 


146 


REFRACTION  AND  HOW  TO  REFRACT. 


used  as  a  coloring-matter  by  glass-blowers.  Cobalt-blue 
glass  comes  in  two  forms  :  one  in  which  the  glass  is  colored 
throughout,  and  the  other  in  which  it  is  colored  on  only  one 
surface,  known  as  "  flashed."  To  the  eye,  cobalt -blue  glass 
appears  dark  blue,  but  contains  a  great  deal  of  red.  For 
purposes  of  testing  ametropia,  a  dark  shade  of  blue  should 
be  selected,  or  two  or  three  pieces  of  a  light  shade  may  be 
cemented  together  so  as  to  give  the  desired  dark  shade. 
This  glass  is  cut  round  and  fitted  into  a  trial-cell.  (See 
Figs.  122  and  123.) 

The  power  of  cobalt-blue  glass  to  exclude  all  but  blue 


FIG.  124. 

and   red  rays  gives  this  test  its  principle.     Blue  rays  being 
/more  refrangible  than  red,  naturally  focus  sooner  than  red. 
~-\  Red  rays  will  focus  back  of  the  blue.     (See  Fig.   1 24.) 

There  are  several  important  details  in  the  use  of  this 
test  which  must  be  carefully  executed  if  definite  results  are 
to  be  obtained  : 

1 .  The  eye  should  be  under  the  influence  of  a  cydoplcgic. 

2.  By  means  of  a   light-screen,  a  small  round  area  of 
steady  white  light  should  be  looked  at  from  a  distance  of 
four  or  six  meters. 

3.  Each  eye  is  to  be  tested  separately. 


ASTIGMATISM. 


147 


FIG.  125 


FIG.  126 


FIG.  127. 


FIG.  128. 


FIG.  129. 


FIG.  130. 


FIG.  131. 


FIG.  132. 


FIG.  133. 


FIG.  134. 


FIG.  135. 


FIG.  136. 


125.  High  hyperopia.  126.  High  myopia.  127.  Low  simple  hyperopic  as- 
tigmatism. 128.  High  simple  hyperopic  astigmatism.  129.  Low  simple 
myopic  astigmatism.  130.  High  simple  myopic  astigmatism.  131.  Low 
compound  hyperopic  astigmatism.  132.  High  compound  hyperopic  astig- 
matism. 133.  Low  compound  myopic  astigmatism.  134.  High  com- 
pound myopic  astigmatism.  135.  Low  mixed  astigmatism.  136.  High 
mixed  astigmatism. 


148        REFRACTION  AND  HOW  TO  REFRACT. 

4.  The  cobalt  glass  may  be  placed  near  the  flame  or, 
better  still,  close   in  front  of  the   patient's  eye ;   in   every 
instance  it  must  be  perpendicular  to   the  front  of  the  eye, 

iand  never  at  an  angle. 

5.  All  other  lights  except  the   one    in  use   should   be 
excluded. 

Diagnosis  of  Emmetropia. — Patient  sees  a  small  circle 
composed  of  two  colors  equally  mixed  ;  purple.  (See  E 
in  Fig.  124.) 

Diagnosis  of  Hyperopia  (see  H  in  Fig.  124). — The 
patient  describes  a  red  ring  of  light  with  a  blue  center. 

Diagnosis  of  Myopia. — The  patient  describes  a  blue 
ring  with  a  red  center.  (See  M  in  Fig.  124.) 

Diagnosis  of  Astigmatism. — If  astigmatic,  then  he  will 
describe  one  of  the  conditions  as  shown  on  page  147.  If 
the  test  is  made  as  suggested,  it  will  have  three  points  of 
recommendation  : 

1.  The  character  of  the  refraction  is  quickly  diagnosed. 

2.  It  may  lead  to  an  early  diagnosis  of  red-blindness,  a 
condition  often  overlooked. 

3.  Likewise  it  will  show  a  central   scotoma  for  red  in 
advanced  toxic  amblyopia,  if  the  eye  is  made  myopic  with 
a  plus  sphere. 

ii.  Thomson's  Ametrometer  (Fig.  137). — This  instru- 
ment has  two  small  gas-flames  about  five  millimeters  in  diam- 
eter, one  stationary  and  the  other  movable  on  a  metal  arm, 
which  can  be  changed  or  revolved  to  any  meridian.  Each 
eye  is  tested  separately  at  a  distance  of  twenty  feet,  and 
preferably  under  a  cycloplegic.  The  method  of  the  test  is 
to  move  one  flame  along  the  metal  arm  until  the  two  lights 
appear  to  fuse.  The  scale,  as  marked  on  the  arm,  gives 
the  approximate  strength  of  lens  necessary  to  correct  the 
ametropia.  By  raising  or  lowering  the  arm  any  meridian 


ASTIGMATISM. 


149 


may  be  tested.     It  is  a   most  ingenious  test,  but  not  in 
common  use.  {/ 

12.  The  Ophthalmometer  (see  Figs.  138  and  139). — 
This  name  literally  means  an  "  eye  measure,"  but  as  the  in- 
strument measures  only  the  different  radii  of  corneal  curva- 
ture, a  much  better  name  would  be  keratometer,  or  measure 
of  the  corneal  radiiv  The  object  of  the  ophthalmometer  is 

h  #- . 


FIG.  137. 

the  measurement  of  corneal  curves  by  means  of  catoptric 
images  viewed  through  a  telescope. 

The  ophthalmometer  consists  of  a  telescope  which  con- 
tains a  Wollaston  birefrangent  prism  placed  between  two  bi- 
convex lenses.  Attached  to  the  telescope  is  a  graduated  arc, 
upon  which  are  placed  two  white  enameled  objects  called 
mires  (targets).  (See  Figs.  139,  140,  141.)  The  left  mire 
is  stationary,  and  is  made  up  of  two_^cm.  squares,  separated 


150 


REFRACTION  AND  HOW  TO  REFRACT. 


by  a  black  line  2  mm.  wide  ;  the  right  mire  is  movable  and 
graduated  into  steps,  each  5  mm.  wide  ;  a  black  line  passes 
through  the  middle  of  these  steps.  For  purposes  of  focus- 
ing, the  telescope  is  mounted  on  a  movable  tripod.  The 
patient  is  seated  with  his  chin  and  forehead  resting  in  a 


FIG.  138. 


frame.  At  the  side  of  the  frame,  and  attached  to  it,  are  two 
or  four  electric  lights  or  Argand  burners,  which  illuminate 
the  mires.  The  surgeon,  looking  through  the  eye-piece  of 
the  telescope,  focuses  the  center  of  the  patient's  cornea 
until  he  sees  two  images  of  each  mire  clearly  ;  then  he 
selects  the  two  central  images  for  further  study  and  ignores 


ASTIGMATISM.  I  5  I 

the  peripheral  images.  The  next  step  is  to  move  the 
right-hand  mire  until  these  two  images  of  the  mires  occupy 
the  center  or  pole  of  the  cornea,  so  that  their  inner  edges 
just  touch  and  the  black  line  in  each  makes  one  continu- 
ous black  line  through  both  (see  Fig.  140)  ;  and  to  do  the 


FIG.  139. 

latter,  the  barrel  of  the  telescope  may  have  to  be  gradually 
revolved  from  left  to  right  or  right  to  left,  but  never  more 
than  45  degrees  either  way.  When  this  position  is  ob- 
tained, the  axis  or  meridian  is  noted  by  the  arrow,  which 
points  to  the  figure  on  the  dial  at  the  back  of  the  arc,  or, 
as  in  some  old  instruments,  on  the  front  of  the  dial.  This 


152        REFRACTION  AND  HOW  TO  REFRACT. 

position  of  the  mires  is  spoken  of  as  the  primary  posi- 
tion. 

Revolving  the  telescope  to  the  opposite  meridian  (mer- 
idian at  right  angles),  which  is  called  the  secondary  posi- 
tion, the  observer  notes  any  change  which  may  have  taken 
place  in  the  relative  positions  of  the  mires.  If  they  have 
not  changed,  but  still  maintain  their  edges  in  apposition,  as 
in  the  primary  position,  then  the  cornea  has  a  uniform  cur- 
vature thoughout,  and  there  is  no  astigmatism  of  the 
cornea  present.  If,  however,  when  the  secondary  position 
is  reached  and  the  catoptric  image  of  the  mires  with  the 
steps  has  encroached  upon  the  catoptric  image  of  the  sta- 
tionary mire,  then  the  astigmatism  is  calculated  by  the 

amount   of  this    overlapping. 
(See  Fig.  141.) 

Each  step  representing  one 
diopter  of  astigmatism,  one- 
half  a  step  of  overlapping 

FIG.  140.  FIG.  141.  would  represent  half  a  diopter, 

etc.     If,  in  making  the  change 

from  the  primary  to  the  secondary  position,  the  mires 
should  separate,  then  the  surgeon  would  know  that  his 
secondary  position  should  have  been  his  primary  position, 
and  he  will  have  to  make  a  corresponding  change. 

As  already  stated,  lenticular  astigmatism  is  not  a  condi- 
tion to  be  ignored,  as  only  too  often  it  will  increase, 
diminish,  or  even  neutralize  corneal  astigmatism,  so  that  in 
point  of  fact  the  ophthalmometric  findings  are  more  often 
useless  than  of  real  value  in  estimating  the  total  refractive 
error.  Cylinders  should  never  be  prescribed  from  the 
ophthalmometric  findings  until  carefully  confirmed  by 
other  and  much  more  reliable  tests.  As  a  keratometer, 
the  instrument  can  not  be  excelled,  and,  therefore,  it  has  a 


ASTIGMATISM.  153 

place  in  testing  the  refraction  in  cases  of  aphakia.  The 
ophthalmometer  as  a  means  of  diagnosis  is  suggestive 
rather  than  positive. 

13.  Estimation  of  Curvature  Ametropia  (Astigma- 
tism) with  the  Ophthalmoscope,  Direct  Method.  —  The 
presence  of  astigmatism  is  diagnosed  by  the  direct  method 
from  the  fact  that  the  vessels  or  details  of  the  fundus  are 
not  all  seen  clearly  with  one  and  the  same  glass  in  the 
ophthalmoscope  ;  in  other  words,  the  vessels  passing  up 
and  down  on  the  disc  are  seen  clearly  with  a  different  lens 
in  the  ophthalmoscope  than  is  required  to  see  the  vessels 
passing  laterally  or  at  right  angles.  (The  amount  of  the 
astigmatism  is  the  difference  in  the  strength  of  the  respective 
lenses  used  for  this  purpose)  for  instance,  if  the  vertical 
vessels  are  seen  best  with  a  -f-4  S.,  and  the  horizontal 
vessels  with  a  -f-2  S.,  then  the  amount  of  the  astigmatism 
would  be  -\-2  D. 

When  using  the  ophthalmoscope  for  making  refractive 
estimates  in  astigmatic  eyes,  /the  student  should  remember 
that  the  glass  with  which  a  vessel  is  seen  distinctly  in  one 
meridian  represents  the  amount  of  the  refraction  in  the 
meridian  at  right  angles  to  this  vesseL)  (In  other  words,  / 
each  vessel  in  the  eye-ground  of  an  astigmatic  eye  is  seen 
clearest  through  the  meridian  at  right  angles  to  its  course.J 
This  is  a  puzzle  to  the  beginner,  but  he  must  remember  that 
cylinders  refract  opposite  to  their  axes.  In  estimating  the 
refraction  with  the  ophthalmoscope,  the  observer  looks  first  (/?  * 
at  the  shape  of  the  disc  ;  if  it  appears  oval,  this  would  be  an 
evidence  of  astigmatism  ;  secondly,  if  the  upper  and  lower 
edges  of  the  disc  are  seen  clearly  with  a  different  strength 
glass  than  that  required  to  see  the  inner  and  outer  margins, 
then  this  would  be  a  further  evidence  of  the  presence  of 
astigmatism  ;  Xut  the  third  and  confirmatory  test  of  the 
presence  of  astigmatism  should  be  the  different  strength 


154         REFRACTION  AND  HOW  TO  REFRACT. 

glasses  required  to  see  the  vessels  distinctly  in  the  neigh- 
borhood of  the  maculay  An  eye  having  an  oval  nerve, 
whose  edges  can  all  be  seen  clearly  with  one  and  the  same 
glass  in  the  ophthalmoscope  is  not  usually  astigmatic. 

Examples  of  estimated  refraction  by  the  direct  method. 

Simple  Hyperopic  Astigmatism. — Vertical  vessels  seen 
with  a  -j-  I  S.  and  horizontal  vessels  seen  without  any  lens 
would  equal  +  1 .00  cyl.  axis  90  degrees. 

Simple  Myopic  Astigmatism. — Vertical  vessels  seen 
without  any  lens  and  horizontal  vessels  seen  with  — 3  S. 
would  equal  — 3  cyl.  axis  180  degrees. 

Compound  Hyperopic  Astigmatism. — Vertical  vessels 
seen  with  -{-4  S.  and  horizontal  vessels  seen  with  +38. 
would  equal  +3.00  S.  O  +  i.oocyl.  axis  90  degrees. 

Compound  Myopic  Astigmatism. — Vertical  vessels 
seen  with  — 2  S.  and  horizontal  vessels  seen  with  — 5  S. 
would  equal  — 2.00  S.  O  — 3.00  cyl.  axis  180  degrees. 

Mixed  Astigmatism. — Vertical  vessels  seen  with 
-(-28.  and  horizontal  vessels  seen  with  — 3  S.  would  equal 
— 3.00  S.  O  +5.00  cyl.  axis  90  degrees. 

14.  Diagnosis  of  the  Character  of  the  Refraction  by 
the  Indirect  Method  (see  Fig.  142  and  p.  99). — There  is 
nothing  exact  about  this  method,  and  the  refractive  error, 
to  be  recognized,  must  be  considerable. 

1.  Gradually  /withdrawing  the  lens  (objective)  from  in 
front  of  the  eye,  if  the  aerial  image  of  the  disc  retains  its 
uniform  size  in  one  meridian,  it  signifies  emmetropia  for  that 
meridian  ;   but  if  it  grows  smaller   in    one  meridian,   that 
meridian  is  hyperopic  ;  or  if  larger,  then  that  meridian  is 
myopic.) 

2.  Ifthe  image  grows  smaller,  but  more  so  in  one  meridian 
than  the  other,  it   signifies  compound  hyperopia.      If  the 
image  grows  larger,  but  more  so  in  one  meridian  than  the 
other,  then  the  condition  is  one  of  compound  myopia.     The 


ASTIGMATISM. 


155 


image  growing  smaller  in  one  meridian,  while  in  the  other 
it  grows  larger,  indicates  mixed  astigmatism. 


FIG.  142.  —  Companion  picture  to  figure  89.  Illustrating  the  indirect  method. 
Rays  from  (lie  lamp  (L)  are  reflected  convergently  from  the  mirror  of  the 
ophthalmoscope,  and,  passing  through  the  convex  lens  and  into  the  eye, 
produce  a  large  retinal  illumination,  extending  from  I  to  I.  TB  are  rays 
from  the  edge  of  the  disc,  and,  leaving  the  eye  parallel,  pass  through  the 
convex  lens  and  form  an  inverted  aerial  image  of  the  disc  at  T/  B'.  The 
-f-4  S.  in  the  ophthalmoscope  magnifies  the  image  T'  B'. 


15.  The  cylinder  lens  test  for  astigmatism  is  described 
under  Applied  Refraction,  page  251. 

16.  Retinoscopy  is  described  in  chapter  vi. 


.          A^<  '• 

%k  G/-&<  v-tfU-W 


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CHAPTER  VI. 
RETINOSCOPY. 
Retinoscopy,  or  the  Shadow  Test. — This  may  be  de- 


FIG.  143. — The  Author's  Schematic  Eye  for  Studying  Retinoscopy. 

fined  as  the  method  of  estimating  the  refraction  of  an  eye 
by  reflecting  into  it  rays  of  light  from  ^a  plane  or  concave 

I56 


RETINOSCOPY. 


157 


mirror,  and  observing  the  movement  which  the  retinal 
illumination  makes  by  rotating  the  mirror. 

Suggestion. — Before  attempting  to  practise  retinoscopy 
upon  the  human  eye,  the  beginner  is  advised  to  study  the 
method  upon  one  of  the  many  schematic  eyes  to  be  found 
in  the  market. 

The  principle  of  retinoscopy  is  the  finding  of  the  point 
of  reversal,  or  myopic  far  point ;  and  when  an  eye  is  emme- 
tropic  or  hyperopic,  it  must  be  given  a  myopic  far  point  by 
means  of  a  convex  sphere.  (Fig.  144.) 

i  METER. 


FIG.  144. 

Advantages  of  Retinoscopy. — 

1.  The  character  of  the  refraction  is  quickly  diagnosed. 

2.  No  expensive  apparatus  is  necessarily  required. 

3.  The  refraction  is  estimated  without  the  verbal  assist- 
ance of  the  patient. 

4.  The  correction  is  quickly  obtained. 

5.  The  value  of  retinoscopy  can  never  be  overestimated 
in  the  young,  in  the  feeble-minded,  the  illiterate  ;   in   cases 

:?of  nystagmus,  amblyopia,  and  aphakia. 

Axiom. — \Yith  an  eye  otherwise  normal  except  for  its 
optic  error,  and  under  the  influence  of  a  reliable  cycloplegic, 
there  is  no  more  exact  objective  method  of  obtaining  its 
refraction  than  by  retinoscopy. 


158 


REFRACTION  AND  HOW  TO  REFRACT. 


The  surgeon  should  wear  any  necessary  correcting 
glasses  and  have  a  vision  of  more  than  —  ;  otherwise  he 
can  never  get  satisfaction  from  this  method.  The  surgeon 
should  keep  his  eyes  wide  open  and  not  hesitate  to  use  his 
accommodation,  as  it  does  not  have  any  effect  on  the  result, 
as  in  estimating  the  refraction  with  the  ophthalmoscope. 


FIG.  145.  FIG.  146. 

Author's  Mirror  with  Folding  Handle. 

FIG.  145. — Showing  central  light  C,  on  small  mirror  B.  This  is  the  light  the 
patient  sees  when  looking  into  the  mirror,  and  corresponds  in  size  to  the 
one-centimeter  opening  in  screen.  D  is  the  folding  cap  handle  to  pro- 
tect B  when  not  in  use.  A  is  the  metal  disc. 

FIG.  146. — Shows  the  light  moved  to  one  side  as  a  result  of  tilting  the  mirror. 

The  patient  must  have  his  accommodation  under  the  in- 
fluence of  a  reliable  cychplegic ;  this  is  imperative.     Each 
eye  is  tested  separately,  and  if  the  patient  has  a  squint,  then 
^^one  eye  should  be  covered  while  its  fellow  is  being  refracted. 
The  patient  must  be  comfortably  seated  and  told  to  look  at 


RETINOSCOPY. 


159 


the   metal   disc  of  the  mirror  or  the  observer's  forehead 
'  above  the  mirror,  and  never  into  the  mirror. 

The  Retinoscope,  or  Mirror. — The  plane  mirror  is  2 
cm.  in  diameter  on  a  round  4  cm.  metal  disc,  with  a  2  mm. 
sight-hole  at  the  center,  made  by  removing  the  silvering, 
and  not  by  cutting  a  hole  through  the  glass.  (See  Figs. 
145  and  146.) 

The  concave  mirror  recommended  has  a  25  cm.  focus 
(ten  inches)  and  is  3  yz  cm.  in  diameter 
on  a  metal  disc  of  the  same  size  as  the 
plane  mirror.  The  sight-hole  is  simi- 
lar in  size  and  made  /n  the  skme  \\  ax- 
as  that  of  the  plane, mirror. 

The  light  should  be  steady,  clear, 
and  \vhite,  and  secured  to  a  movable 
bracket.  For  general  use,  the  Argand 
burner  is  best. 

The  Light-screen,  or  Cover-chim- 
ney.— For  the  purpose  of  intercepting 
the  heat  this  is  made  of  thin  asbestos, 
and  the  iris  diaphragm  attached  to  it 
regulates  the  amount  of  light  desired. 
(See  Fig.  147.) 

The  room  for  retinoscopy  should  be 
darkened  and  all  sources  of  light  except 
the  one  in  use  should  be  excluded. 

Position  of  Light  and  Plane  Mirror. — These  may  be 
as  close  together  as  6  inches  or  as  far  apart  as  6  meters. 
It  is  a  matter  of  choice  with  the  surgeon  himself  where  he 
prefers  to  have  them.  /The  writer  recommends,  however, 
having  the  rays  of  light  come  from  the  10  mm.  opening  in 
the  light-screen,  at  about  6  inches  to  the  left  and  front  of 
the  surgeon,  so  that  the  rays  pass  in  front  of  the  left  eye 


FIG.  147. — Author's  Iris 
Diaphragm  Chimney. 


I6O        REFRACTION  AND  HOW  TO  REFRACT. 

and  fall  upon  the  mirror  held  before  the  right  eye.  Some 
surgeons  prefer  having  the  light,  with  the  3  cm.  opening  in 
the  screen,  placed  over  the  patient's  head  or  to  one  side  of 
it.  (Fig.  148.)  TJic  distance  between  the  light  and  mirror 
will  not  alter  the  direction  of  the  rays  of  light  which  comt 
from  the  patient's  eye. 

Position  of  the  Light  and  the  Concave  Mirror  (Figs. 
148,  149,  150). — As  the  purpose  of  the  concave  mirror  in 
retinoscopy  is  to  focus  rays  of  light  before  they  enter  the 


I  METER 


FIG.  148. — Light  over  Patient's  Head,  and  the  Observer  with  Mirror  at  One 
Meter  Distance. 


patient's  eye,  it  is  always  necessary  to  have  the  light  and 
mirror  widely  separated.  Usually,  the  light  with  the  3  cm. 
opening  in  the  screen  is  placed  to  one  side  or  over  the  pa- 
tient's head,  and  the  surgeon  with  the  mirror  is  seated  about 
one  meter  from  the  patient.  This  will  place  the  focus  of  the 
25  cm.  mirror  about  33  cm.  in  front  of  the  mirror. 

Distance  of  Surgeon  from  Patient. — With  the  plane 
mirror  he  may  approach  within  a  few  inches  of  the  patient's 
eye  to  find  the  point  of  reversal,  but  with  the  concave  mirror 


RETIXOSCOPY. 


161 


he  must  remain  at  a  sufficient  distance  to  have  the  focus  of 
the  mirror  in  front  of  the  patient's  eye. 

How  to  Use  the  Mirror. — It  should  be  held  firmly  in 


FIG.  149. — Illustrating  High  Myopia  with  a  Concave  Mirror. 
Rays  of  light  from  the  lamp  (L)  are  reflected  by  the  mirror  (»//'),  and  form  a 
conjugate  focus  at  L',  and  the  rays  from  this  focal  point  illuminate  the 
retina  at  L1.  Corresponding  effects  result  when  reflection  takes  place 
from  the  mirror  at  m".  The  eye  (E)  behind  the  mirror  recognizes  points 
of  reversal  between  the  eye  and  mirror,  moving  in  the  same  direction  to 
that  in  which  the  mirror  is  tilted. 


FIG.  150. — Illustrating  Hyperopia  with  the  Concave  Mirror. 
The  eye  (E)  recognizes  a  virtual  image  behind  the  eye  under  examination,  so 
that  when  the  mirror  (»/')  is  focusing  the  rays  from  the  lamp  (L)  at  I/, 
the  upper  portion  of  the  retina  is  illuminated,  and  vice  versa,  when  the 
mirror  i  m"  \  is  focusing  the  rays  at  L,,  the  lower  portion  of  the  retina  is 
illuminated.  The  retinal  illumination  moves  opposite  to  that  of  the  mirror. 


the  right  hand  before  the  right  eye,  so  that  the  sight-hole 
is  opposite  to  the  observer's  pupil.     The  movements  im- 


1 62        REFRACTION  AND  HOW  TO  REFRACT. 

parted  to  the  mirror  must  be  limited,  though  they  may  be 
quick  or  slow,  but  never  at  any  time  should  the  mirror  be 
tilted  more  than  2  or  3  mm.,  otherwise  the  light  will  be  lost 
from  the  eye. 

What  the.  Observer  Sees,  or  the  general  appearance 
of  the  reflection  from  the  eye. — The  reflex  from  the  pupil 
varies  in  different  patients,  and  is  subject  to  many  changes 
as  the  refraction  is  altered  by  correcting  glasses,  by  the 
turning  of  the  patient's  eyes,  by  increasing  or  diminishing 
the  distance  between  patient  and  surgeon  or  the  distance 
between  the  light  and  mirror,  or  the  strength  of  the 
light.  The  amount  of  pigment  in  the  eye-ground  will 
change  the  general  appearance  of  the  reflex,  being  dim 
in  some  mulattoes,  and  very  light  in  the  blonde  or  albino. 
If  the  refractive  error  is  a  high  one,  the  reflex  will  appear 
dull  ;  or  if  a  low  error,  it  will  appear  very  bright.  If  the 
•>. media  are  not  clear,  the  reflex  will  be  altered  accordingly. 
The  bright  pin-point  catoptric  images  seen  on  the  cornea 
and  lens  are  not  parts  of  the  test,  and  should  be  avoided  or 
ignored.  The  I  mm.  bright  ring  of  light  sometimes  seen 
at  the  edge  of  the  pupil  should  be  avoided  by  the  beginner 
in  retinoscopy,  as  it  is  an  indication  of  spheric  aberration, 
which  he  will  have  to  consider  after  mastering  other  details 
of  the  method. 

Facial  Illumination. — The  rays  of  light  reflected  from 
the  mirror  illuminate  a  portion  of  the  patient's  face,  and 
always  move  in  the  same  direction  as  that  in  which  the 
mirror  is  tilted,  no  matter  whether  the  mirror  is  plane  or 
concave. 

Retinal  Illumination, — This  corresponds  to  the  portion 
of  the  retina  which  receives  the  rays  of  light  reflected  from 
the  mirror.  The  retinal  illumination  is  also  called  "the 
image,"  "the  light  area,"  etc. 


RETINOSCOPY.  163 

The  Shadow. — This  is  the  non-illuminated  portion  of 
the  retina  immediately  surrounding  the  illumination.  The 
illumination  and  shadow  are,  therefore,  in  contact ;  if  the 
illumination  changes  its  place  upon  the  retina  by  a  move- 
ment of  the  mirror,  then  the  shadow  will  move  also.  By 
this  change  of  illumination  and  shadow  we  speak  of  a 
movement  of  the  shadow. 

Where  to  Look  and  What  to  Look  for. — Rotating  the 
mirror  through  the  various  meridians  of  the  eye,  the 
observer  makes  a  note  of  the  (i)form,  (2)  direction,  and 
;  (3)  rate  of  movement  of  the  retinal  illumination  as  he 
watches  for  them  through  a  four  or  five  millimeter  area  at 
the  apex  of  the  cornea,  as  this  is  the  portion  of  the  refract- 
ive media  in  the  normal  eye  that  the  patient  will  use 
when  the  effects  of  the  cycloplegic  pass  away  and  the 
pupil  regains  its  normal  size. 

Point  of  Reversal. — To  find  the  point  of  reversal  is  the 
underlying  principle  of  retinoscopy.  For  example,  having 
determined  with  the  plane  mirror  at  a  distance  of  one  meter 
that  the  retinal  illumination  moves  with  the  movement  of  the 
mirror  and  a  -f~2-5O  S.  stops  all  apparent  movement  (no 
movement  of  the  illumination  being  seen  and  the  shadow 
having  disappeared),  the  observer  knows  that  his  eye  is  at 
the  point  of  reversal.  Or  with  the  concave  mirror  the 
retinal  illumination  will  move  opposite  to  the  movement  of 
the  mirror  and  will  stop  with  +2.50  S.  before  the  eye. 
The  point  at  which  all  movement  of  the  retinal  illumina- 
tion appears  to  have  ceased  is  the  point  of  reversal. 

The  real  movement  of  the  retinal  illumination  de- 
pends upon  the  mirror — whether  it  is  concave  or  plane. 
With  the  plane  mirror  the  retinal  illumination  always  moves 
with  the  mirror  and  the  light  on  the  face ;  whereas  with  the 
concave  mirror  (focusing  rays  before  they  enter  the  eye), 


164         REFRACTION  AND  HOW  TO  REFRACT. 

the  real  movement  of  the  retinal  illumination  is  always 
opposite  to  that  of  the  mirror.  The  student  should  not 
get  the  real  and  apparent  movements  confused,  but  pay 
close  attention  to  the  apparent  movement. 

Direction  of  the  Apparent  Movement  of  the  Retinal 
Illumination. — With  the  plane  mirror,  the  apparent  move- 
ment of  the  retinal  illumination  will  be  with  the  mirror  and 
with  the  light  on  the  face  as  long  as  the  observer  is  within 
the  point  of  reversal ;  but  just  as  soon  as  the  observer  is 
i  beyond  the  point  of  reversal,  the  retinal  illumination  \\ill 
appear  to  move  opposite  to  the  movement  of  the  mirror 
and  opposite  to  the  movement  of  the  facial  illumination. 


F" 

FIG.  151. 

With  the  concave  mirror  the  apparent  movement  of  the 
retinal  illumination  will  be  with  the  movement  of  the  mir- 
ror and  the  light  on  the  face  as  long  as  the  observer  is  be- 
yond the  point  of  reversal  (Fig.  149) ;  but  just  as  soon  as 
the  observer's  eye  is  within  the  point  of  reversal,  the  retinal 
illumination  will  appear  to  move  against  the  movement  of 
the  mirror  and  against  the  light  on  the  face.  (See  Fig.  1 50.) 

Rate  of  Movement  of  the  Retinal  Illumination. — This 
is  influenced  by  several  factors,  but  practice  will  teach  the 
observer  that  when  the  retinal  illumination  appears  to  move 
slowly,  the  refractive  error  is  a  high  one,  and  when  it  moves 
fast,  the  refractive  error  is  a  low  one. 

Figure    151   represents  a  myopic  eye  with  its  far  point 


RETINOSCOPY. 


i6S 


(point  of  reversal)  at  R/,  and  when  rotating  the  mirror,  this 
point  moves  to  R"  ;  but  if  the  eye  had  its  far  point  at  F', 
and  the  mirror  was  rotated  to  F",  then  the  illumination  at 
R',  having  to  move  through  a  smaller  arc  in  the  same  time, 
appears  to  move  slowly  as  compared  with  F',  which  ap- 
peared to  move  fast.  The  same  condition  is  shown  in 
figure  152,  in  which  the  observer  appears  to  see  an  erect 
virtual  image  back  of  the  retina,  and  R'  appears  to  move 
slowly  as  compared  with  F',  which  appears  to  move  fast. 

Form  of  Illumination.-eA  large,  round  illumination 
may  signify  emmetropia,  hyperopia,  or  myopia,  with  or  with- 
out astigmatism  in  combination.)  Astigmatism  is  recognized 


FIG.  152. 

by  the  presence  of  a  band  of  light,  and  this  band  of  light 
may  be  seen  before  any  correcting  lens  has  been  placed  be- 
fore the  eye  if  the  astigmatic  error  is  high  ;  or  it  will  be 
recognized  during  the  process  of  neutralization  if  the  error 
is  small — /.  e.,  if  the  astigmatism  is  of  low  degree.  )The  pres- 
ence of  astigmatism  is  known,  therefore,  by  the  band  of  light 
or  when  the  illumination  appears  to  move  faster  in  one 
meridian  than  in  the  meridian  at  a  right  angle.  The  astig- 
matism is  in  the  meridian  of  slow  movement. 

The  apparent  difference  between  the  plane  and  con- 
cave mirror  in  the  direction  of  movement  of  the  retinal 
illumination. — \Yith  the  plane  mirror  the  rays  of  light  are 
reflected  as  if  they  came  from  a  point  just  as  far  back  of 


1 66 


REFRACTION  AND  HOW  TO  REFRACT. 


the  mirror  as  the  original  source  of  light  is  in  front  of  it. 
The  surgeon's  eye  behind  a  plane  mirror  is,  therefore,  in 
the  path  of  these  rays,  and  sees  that  portion  of  the  pupil- 
lary area  illuminated  to  which  these  rays  are  directed. 
(See  Fig.  153.) 

With  the  concave  mirror  the  reflected  rays  come  to  a 
focus,  forming  an  inverted  image  of  the  flame,  which  be- 
comes the  immediate  source  of  light  in  front  of  the 
observer's  eye.  When  the  concave  mirror  is  tilted,  the 
immediate  source  of  light  goes  in  the  same  direction,  but 
with  the  result  that  the  opposite  portion  of  the  pupillary 
area  is  illuminated.  (See  Fig.  143.)  This  shows  the 


FIG.  153. 


immediate  source  of  light  at  L'  and  mirror  tilted  downward  ; 
the  rays  proceeding  from  L'  diverge  and  illuminate  the 
upper  portion  of  the  pupillary  area.  Tilting  the  mirror 
upward,  the  immediate  source  of  light  at  L'  moves  upward 
also  (L2),  and  the  lower  portion  of  the  pupillary  area  be- 
comes illuminated.  (See  also  Fig.  149.) 

Rule  for  Neutralizing  Lenses  with  the  Plane  Mirror. 
— When  the  retinal  illumination  appears  to  move  in  the 
same  direction  as  that  of  the  mirror,  the  observer  is  within 
the  point  of  reversal  and  a  plus  lens  must  be  placed  before 
the  eye  to  stop  all  apparent  movement.  When  the  retinal 
illumination  appears  to  move  in  the  opposite  direction  to 


RETINOSCOPY.  1 67 

that  in  which  the  mirror  is  tilted,  the  observer  is  beyond 
the  point  of  reversal,  and  a  minus  lens  must  be  placed 
before  the  eye  to  stop  all  apparent  movement. 

Rule  for  Neutralizing  Lenses  with  the  Concave 
Mirror. — \Yhen  the  retinal  illumination  appears  to  move  in 
the  same  direction  as  that  in  which  the  mirror  is  tilted,  the 
observer  is  beyond  the  point  of  reversal,  and  a  minus  lens 
must  be  placed  before  the  eye  to  stop  all  apparent  move- 
ment. When  the  retinal  illumination  appears  to  move  in 
the  opposite  direction  to  that  in  which  the  mirror  is  tilted, 
the  observer  is  within  the  point  of  reversal,  and  a  plus  lens 
must  be  placed  before  the  eye  to  stop  all  apparent  move- 
ment. 

Rule  for  neutralizing  lenses,  no  matter  whether  the 
mirror  is  plane  or  concave. — When  within  the  point  of 
reversal,  use  a  plus  lens,  and  when  beyond  the  point  of 
reversal,  use  a  minus  lens. 

Application  of  Retinoscopy  in  Ernmetropia  (Fig. 
153). — Rays  of  light  proceed  parallel  from  an  emmetropic 
eye  under  the  influence  of  a  cycloplegic,  and  if  a  -f- 1  S. 
is  placed  in  front  of  such  an  eye,  the  rays  will  converge 
and  form  a  point  of  reversal  at  I  meter  distance,  and  the 
observer  at  this  point  will  not  be  able  to  see  any  move- 
ment of  the  retinal  illumination.  The  same  result  would 
have  been  obtained  at  ^  of  a  meter  if  a  -+-  3  S.  had  been 
used,  or  at  4  meters  if  a  -(-0.25  S.,  or  at  y2  of  a  meter 
if  a  +28.  had  been  used,  etc. 

In  taking  the  patient  from  the  dark-room  to  test  his 
vision  at  6  meters,  an  allowance  must  always  be  made  for 
the  distance  from  the  patient's  eye  at  which  the  point  of 
reversal  was  found.  If  at  ^  of  a  meter,  3  S.  must  be  de- 
ducted from  the  lens  used  ;  if  at  ^  of  a  meter,  48.;  if  at 
6  meters,  nothing,  or  o.  ij2._ 


i68 


REFRACTION  AND  HOW  TO  REFRACT. 


Application  of  Retinoscopy  in  Hyperopia. — The 
x'same  conditions  hold  good  in  hyperopia  as  in  emmetropia. 
If  a  +4  S.  gives  a  point  of  reversal  at  one  meter,  then 
I  S.  must  be  taken  from  the  4  S.  to  give  the  eye  parallel 
rays  of  light,  or  infinity  vision.  If  a  +48.  gave  a  point 
of  reversal  at  2  meters,  then  0.50  S.  would  have  to  be 
deducted  from  the  4  S.  for  the  infinity  correction,  which 
would  be  -+-3.50  S. 

Application  of  Retinoscopy  in  Myopia. — Rays  of  light 
from  a  myopic  eye  come  to  a  focus  at  some  point  inside  of 
infinity,  and  if  the  surgeon  so  desires,  he  may  approach  such 
an  eye  from  a  distance  of  six.  meters,  until  he  finds  a  point 
where  the  retinal  illumination  ceases  to  move  (where  it  does 
not  appear  to  move)  ;  and  then,  measuring  this  distance  from 
the  eye  under  examination,  he  can  quickly  calculate  the 
amount  of  the  myopia.  This  can  not  be  done  with  the 
concave  mirror  if  the  myopia  is  more  than  2  S.  If  the 
reversal  point  is  at  4  meters,  3  meters,  2  meters,  I  meter, 
*4  of  a  meter,  ^  of  a  meter,  or  ^  of  a  meter,  then  the 
myopia  would  be  0.25  S.,  0.33  S.,  0.50  S.,  I  S.,  2  S.,  3  S., 
4  S.,  respectively. 

If  the  surgeon  will  always  refract  the  patient's  eyes  so 
that  he  gets  the  point  of  reversal  at  I  meter  distance,  he 
will  have  the  following  rule  to  guide  him — /.  c.  : 
.,   To  add  a  — I   sphere  to  the  dark-room  correction,  no 
matter  what  that  may  be.     For  example  : 

Dark-room,         o.oo        40.258.    40.508.    -(-0.758.    4-I-°°S.  -(-1.25  S. 
Add,     .    .    .  — i.ooS.    — 1.008.    — 1.008.    — i.ooS.    — i.ooS.  — i.ooS. 


Infinity,    .    . — i.ooD.  —0.750.  — 0.50 D.  — 0.25!).       o.oo       40.25  D. 

Application  of  Retinoscopy  in  Astigmatism. — If  the 
surgeon  has  mastered  retinoscopy  in  hyperopia  and  myopia, 
he  should  not  have  any  difficulty  in  pursuing  exactly  the 


RETINOSCOPY.  169 

same  course  in  cases  of  astigmatism.     As  already  stated, 

the  presence  of  astigmatism   is  diagnosed   by  the  presence 

of  a  band  or  ribbon-like  streak  of  bright  illumination  which 

extends  across  the  pupillary  area. 

(See  Fig.  154.)    This  band  of  light 

may  be  seen  before  any  neutralizing 

lens  is  placed  in   front  of  the  eye, 

if  the  astigmatism  is  in   excess   of 

the    spheric    correction,   as   in    the 

following  formula  :  I  L 

+0.75  sph.  C  +4-50  cyl.  axis  105  degrees.        Fl.G-  .I54-~ Band  of  Light. 

Astigmatism   Axis    90  de- 
— i.oo  sph.  ^j — 5-OO  cyl.  axis  165  degrees.  aree 

Or  the  presence  of  astigmatism 

may  not  be  recognized  until  after  a  sphere  has  been  placed 
in  front  of  the  eye,  as  in  one  of  the  following  formulas  :     ,.-,,• 

+  4.50  sph.  C  +0.75  cyl-  axis    75  degrees. 
— 5.00  sph.  ^ — i-oo  cyl.  axis  180  degrees. 

In   refracting  cases  of  astigmatism  with  the  retinoscope, 
[all  the  surgeon  has  to  do  is  to  refract  the  meridian  of  least-'' 
ametropia  first,  and  then  the  meridian  of  greatest  ametropia.         ^*^ 
Taking  the  following  formula  : 

-(-2.50  sph.  ;3  -f  i.oo  cyl.  axis  90  degrees  ; 

in  the  dark-room  a  +3-5O  S.  would  make  all  movement 
cease  in  the  vertical  meridian,  at  one  meter  distant ;  but 
when  the  mirror  is  tilted  in  the  horizontal  meridian,  there 
would  be  seen  a  band  of  light  extending  across  the  pupil 
on  axis  90  degrees.  Then,  substituting  +4.50  S.  for  the 
3.50  S.,all  movement  will  cease  in  the  horizontal  meridian, 
a  +4.50  S.  neutralizing  the  horizontal  meridian.  The 
difference  between  these  two  spheres  is  I  D.,  which  is  the 
amount  of  the  astigmatism.  In  neutralizing  astigmatism 
•ppT-the  writer  advises  using  spheres,  and  after  each  meridian 


I/O 


REFRACTION  AND  HOW  TO  REFRACT. 


has  been   refracted,  to   make  the  cylindric  correction,  and 
prove  it,  if  so  desired. 

Axonometer. — To  find  the  exact  axis  subtended  by  the 
band  of  light  while  studying  the  retinal  illumination,  when 
the  meridian  of  least  ametropia  has  been  corrected,  the 
writer  has  suggested  a  small  instrument,  which,  for  want  of 
a  better  name,  he  has  called  an  axonometer.  This  is  a 
black  metal  disc,  with  a  milled  edge,  I  ^  mm.  in  thickness, 
of  the  diameter  of  the  ordinary  trial-lens,  and  mounted  in 
a  cell  of  the  trial-set.  It  has  a  central  round  opening,  12 


FIG.  155. 


mm.  in  diameter — the  diameter  of  the  average  cornea  at  its 
base.  Two  heavy  white  lines,  one  on  each  side,  pass  from 
the  circumference  across  to  the  central  opening,  bisecting 
the  disc.  To  use  the  axonometer,  place  it  in  the  front 
opening  of  the  trial -frame,  and  with  the  patient  seated  erect 
and  frame  accurately  adjusted,  so  that  the  cornea  of  the  eye 
to  be  refracted  occupies  the  central  opening.  As  soon  as 
that  lens  is  found  which  corrects  the  meridian  of  least 
ametropia,  and  the  band  of  light  appears  distinct,  turn  the 


h. 


RETINOSCOPV.  I/I 

axonometer  slowly  until  the  two  heavy  white  lines  accu- 
rately coincide,  or  appear  to  make  one  continuous  line  with 
the  band  of  light.  (See  Fig.  155.) 

The  degree  mark  on  the  trial-frame  to  which  the  arrow- 
ead  at  the  end  of  the  white  line  then  points  is  the  exact 
axis  for  the  cylinder. 

Application  of  Retinoscopy  in  Mixed  Astigmatism. — 
Here  the  dark-room  correction  (after  making  deductions  for 
the  distance  of  the  point  of  reversal)  will  show  one  meridian 
myopic"  and  the  other,  at  right  angles  to  it,  as  hyperopic.  If 
the  astigmatism  is  more  than  one  diopter,  in  each  meridian, 
the  surgeon  will  diagnose  in  the  dark-room  the  condition 
of  mixed  astigmatism  by  opposite  movements  in  the  me- 
ridians of  minimum  and  maximum  ametropia. 

Application  in  Irregular  Astigmatism. — This  condi- 
tion is  either  in  the  lens  or  cornea,  usually  in  the  latter. 
The  reflex  is  more  or  less  obscured  by  areas  of  darkness, 
which  make  it  extremely  difficult  to  study  the  refraction, 
and  the  observer  will  have  to  change  his  distance  repeat- 
dly  to  find  clear  spaces  as  close  to  the  center  of  the  pupil 
as  possible,  as  it  is  this  portion  of  the  pupillary  area  that 
the  patient  will  see  through  when  the  mydriatic  effect  passes 
away.  The  kaleidoscopic  picture  obtained  by  moving  the 
mirror  so  as  to  describe  a  circle  at  the  periphery  of  the 
pupillary  space  is  quite  diagnostic  of  the  corneal  condition. 
\Yhatever  correction  is  obtained  should  be  kept  for  reference 
in  a  postcycloplegic  manifest  refraction,  as  it  will  not  always 
do  to  order  the  glasses  while  the  eye  has  its  pupil  dilated. 
The  patient  may  choose  a  slightly  different  correction  in 
such  cases,  after  the  pupil  regains  its  accustomed  size. 

Irregular  Lenticular  Astigmatism. — This  is  often  more 
uniform  than  the  corneal  variety,  and  is  characterized  by 
faint  striae  in  the  lens,  pointing  in  toward  the  center.  If 


REFRACTION  AND  HOW  TO  REFRACT. 


the  striae  are  not  very  faint,  they  may  be  recognized  with  the 
ophthalmoscope,  even  before  any  cycloplegic  has  been  used, 
issor   Movement   (see    Fig.    156). — This   is  a  condi- 
i^/*tion  in  which  two  bands  of  light  are  present,  usually  in 
the  horizontal  meridian  or  inclined  a  few  degrees  therefrom. 
^>»v^*      Tilting  the  mirror  in  the  vertical   meridian,  a  band  of  light 
^^0.  3    is  seen  to  come  from  above  and  to  meet  another  band,  which 
comes  from  below  ;  while  these  two  bands  are  approaching, 
the  dark  space  between  them  gradually  disappears,  until  the 

two  bands  unite  and  form  one  band 
across  the  pupil  in  or  approximat- 
ing the  horizontal  meridian.  This 
movement  of  the  bands  is  likened 
to  the  action  of  the  blades  of  a  pair 
of  scissors,  and  hence  the  name. 
To  refract  a  case  of  this  character, 
the  observer  must  proceed  slowly 
and  endeavor  to  neutralize  the 
horizontal  meridian  first,  and  then 

add    minus    cylinders  with    the    avi^    ^ojp^ponrting    tr>    fhp 


FIG.    156. — Scissor    Move- 
ment 


axis  of 


two 


The  resulting  prescription  should 
also  be  a  plus  sphere  with  a  minus  cylinder,  the  cylinder 
of  less  strength  than  the  sphere,  asthe  condition  is^notjme 
of  mixed  astigmatism,  the  patient  preferring  this  combina- 
tion, as  a  rule. 

Conic  Cornea.  —  In  this  condition  the  observer  is  im- 
pressed at  once  with  the'  bright  central  illumination,  which 
usually  moves  opposite  to  the  movement  of  the  peripheral 
illumination.  ^The  best  way  to  neutralize  a  case  of  this 
character  is  to  proceed  as  in  a  case  of  irregular  astigmatism. 
The  observer  should  also  be  on  the  lookout  for  a  band  of 
light  in  this  central  illumination,  as  most  of  these  cases  are 
astigmatic. 


RETINOSCOPY. 


173 


Spheric  Aberration. — This  is  of  two  kinds — positive 
and  negative.  (See  Figs.  157  and  158.)  In  the  positive 
form  the  peripheral  refraction  (A,  A,  that  at  the  edge  of  the 
pupil)  is  stronger  than  the  central  (B,  B) ;  the  reverse  of 

is  condition,  negative  aberration,  is  seen  in  conic  cornea. 
These  two  varieties  of  refraction  should  not  worry  the 
observer,  as  most  of  the  peripheral  aberration  is  covered 
up  by  the  iris  when  mydriasis  passes  away,  and,  therefore, 
is  not  of  any  great  moment,  except  in  conic  cornea. 


FIG.  157. — Positive  Aberration. 


Fie.  158.  —  Negative  Aberration. 


he  Reisner  Retinoscope  (Figs.  159  and  160).  —  This 
instrument  combines  the  mirror  with  an  axis  finder.  The 
metal  disc,  on  which  the  mirror  is  secured  by  means  of  a 
coiled  spring  at  one  point  only,  has  a  milled  edge  like  that 
of  an  ophthalmoscope.  (See  Fig.  159.)  At  the  junction 
of  the  metal  disc  with  the  handle  is  a  small  push-button 
which  has  a  short  metal  pointer  which  extends  upward  and 
beneath  the  mirror.  Figure  160  is  a  back  view  of  the  in- 
strument, and  shows  the  degree-marks  of  half  a  circle  and 
an  index  or  pointer.  When  beginning  the  examination, 


1/4        REFRACTION  AND  HOW  TO  REFRACT. 

this  instrument  may  be  used  just  the  same  as  any  other 
retinoscope,  but  as  soon  as  the  surgeon  sees  a  band  of  light 
tli en  he  presses  the  push-button  to  see  if  the  mirror  rotates 
with  its  axis  corresponding  to  the  long  measurement  of  the 
band  of  light.  If  this  is  not  so,  then  the  mirror  must  be 
turned  by  means  of  the  milled  edge,  using  the  index-finger 
as  in  turning  the  disc  of  an  ophthalmoscope.  When  the 
axis  of  rotation  of  the  mirror  corresponds  with  the  long 


FIG.  159. — (One-half  size.)  FIG.  1 60. —  (One-half  size.) 

axis  of  the  band  of  light,  then  the  pointer  on  the  back  of 
the  instrument  indicates  the  axis  of  the  astigmatism,  and 
now  all  the  surgeon  has  to  do  as  he  makes  the  changes  in 
the  lenses  is  to  press  the  button  only.  The  handle  of  the 
retinoscope  is  now  held  perfectly  still  and  the  upper  portion 
of  the  disc  rests  firmly  against  the  observer's  brow.  The 
merits  of  this  ingenious  instrument  are  as  follows :  The 


RETINOSCOPY.  1/5 

handle  is  held  perfectly  still,  the  mirror  alone  does  the 
moving;  the  sight-hole  is  always  in  front  of  the  pupil;  the 
axis  of  the  astigmatism  is  carefully  recorded. 


FIG.  161. — (Two-thirds  size. )  FIG.  162. — (Two-thirds  size.) 


rer 

Th 


e   Luminous   Retinoscope  (Figs.   161    and    162).— 
DcZcng  Patent.  —  The  latest  improvement  in  retinoscopes  is 


1/6        REFRACTION  AND  HOW  TO  REFRACT. 

the  luminous  instrument  here  described.  This  instrument 
is  the  author's  plane  mirror  with  the  electric  light  attach- 
ment. (Fig.  1 6 1.)  The  filament  is  contained  in  a  tube 
placed  at  an  angle  of  45  degrees  with  the  handle  and  the 
mirror  is  correspondingly  tilted  at  an  angle  of  22  degrees. 
The  light  from  the  filament  passes  divergently  to  a  strong 
convex  lens  which  renders  the  rays  less  divergent  as  they 
fall  upon  the  mirror,  and  from  the  mirror  the  rays  pass 
divergently  to  the  patient's  eye.  (See  Fig.  162.)  This  in- 
strument has  innumerable  points  of  merit:  It  does  away 
with  any  use  of  gas  or  lamp  or  cover  chimney;  the  ob- 
server is  not  annoyed  with  the  heat  from  the  gas  or  lamp ; 
the  observer  does  not  have  to  move  the  light  or  bracket 
when  changing  from  one  distance  to  another  as  when  work- 
ing with  the  gaslight  close  to  the  mirror ;  the  electric  wires 
(cords)  carrying  the  current  to  the  filament  are  of  sufficient 
length  to  give  the  observer  two  meters  of  space  in  which 
to  practise  the  method;  the  brilliancy  of  the  illumination 
can  be  made  most  intense  or  diminished  very  materially 
with  a  convenient  rheostat;  the  size  of  the  divergent  pencil 
may  also  be  controlled  by  adjusting  the  condensing  lens  at 
the  end  of  the  tube. 


CHAPTER    VII. 
MUSCLES. 

EXAMINATION  OF  THE  EXTERNAL  EYE  MUSCLES. 

General  Considerations. — When  the  retinal  image  of  an 
object  is  situated  exactly  on  the  fovea,  the  eye  is  said  to 
"fix"  the  object. 

Normally,  when  b^oth  eyes  "fix  "  the  object,  each  eye 
has  an  image  of  the  object  on  its  fovea,  and  these  foveal 
images  or  impressions  are  transmitted  to  the  brain  and  fused 
as  one  image  in  the  visual  centers.  This  condition  is  spoken 
of  as  equipoise,  or  orthophoria,  and  the  eyes  are  said  to  be 
in  equilibrium,  or  to  balance.  Whenever  one  eye  alone  fixes 
an  object,  and  the  fellow-eye  receives  the  image  of  the  same 
object  on  a  part  of  its  retina  distant  from  the  fovea,  then 
the  brain  takes  note  of  two  separate  impressions,  and  this 
condition  is  spoken  of  as  double  vision  (diplopia). 

(rt)  The  image  of  an  object  formed  upon  the  retina  above 
the  fovea  is  projected  downward — /.  c.t  objects  situated 
below  the  horizontal  line  of  vision  are  recognized  by  that 
portion  of  the  retina  above  the  fovea. 

(&)  The  image  of  an  object  formed  upon  the  retina  below 
the  fovea  is  projected  upward — i.  e.,  objects  situated  above 
the  horizontal  line  of  vision  are  recognized  by  that  portion 
of  the  retina  below  the  fovea. 

(r)  The  image  of  an  object  formed  on  the  retina  to  the 
nasal  side  of  the  fovea  is  projected  toward  the  temporal 
side — /.  c.,  objects  to  the  temporal  side  have  their  images 
formed  upon  the  nasal  portion  of  the  retina. 

177 


REFRACTION  AND  HOW  TO  REFRACT. 

(d)  The  image  of  an  object  formed  on  the  retina  to  the 
temporal  side  of  the  fovea  is  projected  toward  the  nasal  side 
— /'.  c.,  objects  to  the  nasal  side  have  their  images  formed 
upon  the  temporal  portion  of  the  retina. 

Homonymous  Diplopia  (Greek,  ^xovurw-;  from  «/*«?, 
same,  and  Zw<ia,  name). — Figure  163  shows  the  right 


FIG.  163. 

eye  (R)  fixing  upon  the  object  (O),  but  the  left  eye  is 
turned  inward,  so  that  rays  from  O  fall  upon  its  retina  to  the 
nasal  side  of  the  fovea  (M),  and  are  projected  outward  to 
the  temporal  side  ;  the  result  is  that  the  left  eye  sees  a  false 
object  to  the  left  of  the  real  object.  This  condition  of  the 
objects  is  spoken  of  as  homonymous  diplopia. 


MUSCLES. 


179 


Heteronymous  Diplopia  (Greek,  ere/™?,  other ;  and  ^ 
name). — Figure  164  shows  the  right  eye  fixing  the  object 
(O),  but  the  left  eye  is  turned  outward,  so  that  rays  from  O 
fall  upon  the  retina  to  the  temporal  side  of  the  fovea  and 
are  projected  to  the  nasal  side,  with  the  result  that  the  left 
eye  sees  a  false  object 
to  the  right  of  the  real 
object.  This  condi- 
tion of  the  objects  is 
spoken  of  as  heter- 
onymous  or  crossed 
diplopia. 

H  y  p  e  r  p  horia 
(Greek,  "~£/^  over, 
jiboj^g. ;  ^<y>efv,  to 
tend). — In  the  con- 
sideration of  vertical 
diplopia,  —  which  is 
always  a  condition 
of  crossed  diplopia, 
never  homonymous 
diplopia,  — h  the  eye 
which  is  deviated  up- 
ward is  spoken  of  as 
the  hyperphoric  eye,x> 
and  necessarily  its 
image  must  be  lower 

than  its  fellow.  For  instance,  if  the  left  eye  fixes  an  object 
and  the  right  eye  is  turned  upward,  the  rays  of  light  from 
the  object  would  fall  upon  the  upper  part  of  the  retina  of  the 
right  eye,  and  would  be  projected  downward  below  the  true 
object ;  and  this  position  of  the  right  eye  is  spoken  of  as 
right  hypcrphoria.  Or  if  the  right  eye  fixes  an  object  and 


O'M 


FIG.  164. 


ISO        REFRACTION  AND  HOW  TO  REFRACT. 

the  left  eye  sees  a  false  object  below,  then  the  position  of 
the  left  eye  is  spoken  of  as  left  hyperphoria.  Unfortunately, 
in  hyperphoria  (unless  from  paralysis)  the  position  of  the 
eyes  does  not  tell  whether  the  right  superior  rectus  is  too 
strong  and  the  left  inferior  rectus  too  weak,  or  the  left  supe- 
rior rectus  too  weak  and  the  right  inferior  rectus  too  strong. 

Muscle  Phorometry.  —  Testing  the  power  of  the  ocular 
muscles. 

Abduction.  —  The  power  of  the  external  recti  muscles  to 
turn  the  eyes  outward.  The  patient  is  comfortably  seated 
and  told  to  look  at  a  point  of  steady  light  at  a  distance  of 
about  6  meters,  slightly  below  the  level  of  his  eyes,  never 
above  the  level.  In  this  position  prisms  with  their  bases  in- 
!ward  are  placed  in  front  of  one  or  both  eyes  until  the 
patient  says  he  sees  two  lights  very  close  together.  The 
strength  of  the  prism  or  prisms  thus  placed  before  the 
eyes  which  will  just  permit  the  eyes  to  see  one  object 
and  if  increased  would  produce  diplopia,  represents  the 
power  of  the  external  recti  muscles.  This  is  spoken  of  as 
the  power  of  abduction,  and  is  abbreviated  Abd.  For 
example,  if  with  7  centrads,  base  in,  before  the  eyes  there 
are  two  lights,  and  with  6  centrads  there  is  only  one  light, 
then  6  centrads  would  represent  the  amount  of  the  abduc- 
tion. In  other  words,  in  the  case  supposed,  as  long  as 
there  is  less  than  7  centrads  before  the  eyes,  base  inward, 
the  external  recti  muscles  can  overcome  their  effect,  but  as 
soon  as  a  prism  stronger  than  6  centrads  is  used,  then  the 
external  recti  muscles  can  not  counteract  the  effect,  and 
diplopia  is  the  result. 

Adduction.  —  The  power  of  the  internal  recti  muscles  to 
turn  the  eyes  inward.  The  power  of  the  internal  recti  is 
tested  in  the  same  way  as  the  external,  except  that  the 
prism  is  placed  base  outward.  This  is  spoken  of  as  adduc- 


T'  I      I    j,   1^     '\S-*- 

)        \S^\  X         V— «C/i 

.     ' 

) 


MUSCLES.  iSl 

tion,  and  is  abbreviated  Add.  For  example,  if  with  19 
centrads,  base  out,  before  the  eyes  two  lights  are  seen,  and 
with  1 8  centrads  only  one  light,  then  18  centrads  represent 
the  power  of  adduction.  In  other  words,  as  long  as  there  is 
a  prism  of  18  or  less  than  18  centrads  before  the  eyes,  base 
outward  in  this  case,  the  internal  recti  muscles  can  over- 
come the  effect ;  but  as  soon  as  a  prism  stronger  than  1 8 
centrads  is  used,  then  the  internal  recti  muscles  can  not 
counteract  the  effect,  and  diplopia  is  the  result.  It  must 
be  remembered  that  the  internal  and  external  recti  are 
antagonistic,  and  that  the  muscles  of  the  two  eyes  are 
tested  together.  The  relative  power  of  adduction  to  abduc- 
tion has  been  variously  estimated,  but  most  authorities  are 
agreed  that^adduction  is  about  three  times  that  of  abduc- 
tion, or  about  3  to  I — that  is  to  say,  in  eyes  with  normal 
muscle  balance,  if  adduction  is  represented  by  18  centrads, 
then  abduction  should  be  6  ;  or  if  adduction  is  represented 
by  24  centrads,  then  abduction  should  be  8  centrads  ;  or  if 
adduction  is  1 2  centrads,  then  abduction  should  be  4  cen- 
trads, etc^ 

Sursumduction. — This  is  the  power  of  the  eyes  to  fuse 

AVO  images  when  one  eye  has  a  prism  placed  base  up  or 

down  before  it.      For  example,  if  a  3^   centrad  prism  is 

placed  base  up   or   down   before   either   eye   and    diplopia 

results  and  persists,  and  then  a  3  centrad  is  substituted  and 

there  is  no  diplopia,  then  the  eyes  have  overcome  the  effect 

of  the  prism  and  the  amount  of  the  sursumduction  is  said 

'  to  be  3  centrads.     This  test  for  sursumduction  is  made  at 

the  same  distance  as  in  testing  the  lateral  muscles.      In 

health  the  power  of  the  superior  and  inferior  recti  muscles 

is,  as  a  rule,  the  same — that  is  to  say,  they  antagonize  each 

other  equally.   The  power  of  the  superior  recti  is  spoken  of 

I  as  supraduction,  snrsninvergencc  ;   and  that  of  the  inferior 

;  recti,  as  infraduction,  dc.i>rsnnii'crgcncc. 

• 


1 82        REFRACTION"  AND  HOW  TO  REFRACT. 

Muscular  Imbalance. — Whenever  there  is  any  disturb- 
ance in  the  power,  strength,  or  force  of  the  ocular  muscles, 
the  condition  is  no  longer  one  of  equipoise,  or  equilibrium, 
or  muscle  balance,  but  is  spoken  of  as  muscular  imbalance 
(heterophoria).  From  this  statement  it  must  not  be  sup- 
posed that  the  two  eyes  can  not  simultaneously  "  fix  "  an 
object,  any  more  than  it  must  be  supposed  that  a  hyperoptc 
eye  can  not  see  or  have  — ..-  vision '  without  correcting 
glasses. 

Just  as  in  hyperopia  distant  vision  may  be  made  clear 
by  the  effort  of  accommodation,  so  in  muscular  imbalance 
the  visual  axes  can  be  directed  to  one  point  of  fixation  by 
increased  innervation.  Muscular  imbalance  is  subdivided 
into  two  classes — insufficiency  and  strabismus. 

The  following  nomenclature  of  muscular  anomalies,  sug- 
gested by  Stevens,  of  New  York,  is  in  common  use  : 
OrtJiopJioria,  perfect  muscle  balance,  equipoise,  or  binocular 

equilibrium. 

Orthotropia,  perfect  binocular  fixation. 

Hctcroplwria,  imperfect  binocular  balance,  or  imperfect  bin- 
ocular equilibrium. 
Hcterotropia,  a  squint  or  decided  deviation  or  turning  from 

parallelism. 

Hypcrplioria,  ^tendency  of  one  eye  to  deviate  upward. 
Hypertropia,  a  deviation  of  one  eye  upward. 
Esophoria,  a  tendency  of  the  visual  axes  to  deviate  inward. 
Esotropia,  a  deviation  of  the  visual  axes  inward. 
Exophoria,  a  tendency  of  the  visual  axes  to  deviate  outward. 
Exotropia,  a  deviation  of  the  visual  axes  outward. 
Hypercsophoria,  a  tendency  of  the  visual  axis  of  one  eye 

to  deviate  upward  and  inward. 
Hypercsotropia,  a  deviation  of  the  visual   axis  of  one  eye 

upward  and  inward. 


MUSCLES.  183 

Hypercxophoria,  a   tendency  of  the  visual   axis  of  one  eye 

to  deviate  upward  and  outward. 
Hypcrexotropiat  a  deviation  of  the  visual  axis  of  one  eye 

upward  and  outward. 

Insufficiency. — Also  called  latent  deviation,  hetero- 
phoria,  or  latent  squint.  This  may  be  defined  as  the  con- 
dition in  which  there  is  a  tending  or  tendency  of  the  visual 
axes  to  deviate  from  the  point  of  fixation  ;  this  may  be  slight 
or  transitory. 

Causes  of  Insufficiency. — The  chief  cause  of  insuffi- 
ciency is  some  form  of  ametropia.  Another  cause  may  be 
an  anatomic  defect  of  one  or  more  of  the  ocular  muscles 
themselves,  or  a  weakness  of  the  muscle  or  muscles  indi- 
vidually, or  as  a  result  of  some  systemic  weakness.  The 
ocular  muscles  often  sympathize  with  the  economy. 

Symptoms  of  Insufficiency,  or  Muscular  Asthenopia. 
— Accommodative  and  muscular  asthenopia  are  intimately 
associated,  and  the  latter  is  so  often  the  companion  of  the 
former  that  they  produce  symptoms  which  are  identical  in 
both  and  make  it  difficult  to  draw  any  sharp  line  of  demar- 
cation between  the  two.  In  muscular  asthenopia,  how- 
ever, the  patient  complains  that  the  eyes  "become  weak" 
or  "tired"  after  any  prolonged  use,  and  that  this  is  espe- 
cially apt  to  occur  by  artificial  light  ;  that  nearby 
objects  (reading,  writing,  or  sewing)  grow  dim  ;  that  the 
words  "seem  to  jump,"  or  the  "letters  run  together,"  and 
in  some  cases  occasionally,  and  in  others  more  frequently, 
objects  appear  double  for  a  moment.  Sometimes  one  of 
the  eyes  feels  as  if  it  \vas  turning  outward  or  inward. 
There  are  innumerable  reflex  symptoms,  dizziness,  nausea, 
vomiting,  fainting,  and,  in  some  instances,  "  all  becomes 
dark  for  a  minute."  Such  patients  often  become  very 
anxious,  fearing  sudden  blindness,  etc. 


184        REFRACTION  AND  HOW  TO  REFRACT. 

Diagnosis  or  Tests  for  Insufficiency  (Heterophoria). — 
Before  taking  up  the  individual  tests  for  insufficiencies,  it  is 
well  for  the  observer  to  study  the  movements  or  excursions 
of  the  eyes ;  and  to  do  this  the  patient,  with  his  head  erect 
and  steady  in  one  position,  fixes  with  his  eyes  the  point  of 
a  pencil  held  in  the  hand  of  the  observer  at  about  thirteen 
inches  distant.  The  pencil  is  moved  from  left  to  right  and 
from  right  to  left,  and  upward  and  downward  ;  as  this  is 
done,  the  surgeon  should  watch  closely  to  see  that  each 
eye  has  a  normal  mobility  and  the  two  eyes  move  together. 
From  a  central  point  of  fixation  the  eyes  should  move 
inward  about  45  degrees,  outward  45  or  50  degrees, 
upward  about  40  degrees,  and  downward  about  60  degrees. 
The  tropometer  of  Stevens  will  estimate  the  limit  of  motion 
of  each  eye  separately,  but  if  there  is  a  defect  in  mobility, 
the  surgeon  may  recognize  it  by  comparing  the  distance  of 
the  corneal  edge  in  each  eye  from  a  certain  definite  fixed 
point ;  for  instance,  whether  the  lid  margins  encroach 
equally  upon  the  cornea  or  have  equal  intervals  between 
cornea  and  lid  edges. 

The  Cover  Test. — The  patient  is  told  to  look  at  the 
point  of  a  pencil  held  in  the  hand  of  the  surgeon  on  a  level 
with  the  patient's  eyes  in  the  median  line,  and  distant  about 
eighteen  inches,  or  at  an  object  six  meters  distant.  While 
the  eyes  fix  the  point  of  the  pencil  or  distant  object,  the 
surgeon  covers  one  eye  with  a  small  card,  and  a  moment 
later  quickly  withdraws  it  and  observes  the  position  and 
movement  of  the  eye  which  he  has  just  uncovered  ;  if  it 
moved  inward  toward  the  nose  to  fix  the  point  of  the  pen- 
cil, then  there  must  have  been  an  outward  tendency  of  that 
eye  when  under  cover  ;  in  other  words,  the  external  muscles 
must  have  been  strong  or  the  internal  weak.  If  the  eye 
thus  released  from  the  cover  had  moved  outward  toward 


MUSCLES.  185 

the  temple  to  fix  the  point  of  the  pencil,  then  the  external 
recti  must  have  been  weak  or  the  internal  strong.  If  the 
eye  released  from  cover  goes  up  to  fix.  then  the  fellow-eye 
deviates  upward,  and  vice  versa.  This  test  is  not  always 
reliable,  and  yet  it  may  be  a  guide  to  further  study. 

The  Fixation  Test. — Instead  of  covering  one  eye,  as  in 
the  previous  test,  the  patient  "  fixes  "  the  point  of  the  pencil 
as  it  is  slowly  advanced  in  the  median  line  toward  the  nose, 
'up  to  within  four  inches,  if  necessary.  During  this  advance 
of  the  pencil,  if  there  is  a  weakness  of  the  interni,  the  eye 
with  the  weaker  internus  is  the  one  which  will  usually 
deviate  outward. 

To  Determine  Lateral  Insufficiency. — The  condition 
in  which  there  is  either  a  tendency  for  the  visual  axes  to 
deviate  outward  (exophoria),  or  a  tendency  for  the  visual 
axes  to  deviate  inward  (esophoria).  Proceed  by  producing 
"  vertical  diplopia.  Place  a  ten  centrad  prism  base  down 
before  one  eye, — for  instance,  the  right  eye, — and  have  the 
patient  look  at  a  point  of  light  on  a  level  with  his  eyes  at  a 
distance  of  six  meters.  He  will  see  two  lights,  one  above 
the  other  ;  the  upper  light  must  belong  to  the  right  eye, 
because  the  prism  before  the  right  eye  bent  the  rays  down- 
ward. If  one  light  is  directly  above  the  other,  then  the 
condition  is  presumably  one  of  equilibrium  or  equipoise. 

If  the  upper  light,  however,  is  to  the  right,  then  the 
visual  axes  deviate  inward  (esophoria).  The  amount  of 
the  esophoria  (insufficiency  of  the  external  recti)  is  repre- 
sented by  that  prism  placed  base  outward  before  the  left 
eye  which  will  bring  one  light  directly  above  the  other.  If 
the  upper  light  had  been  to  the  left,  then  there  would  have 
been  a  tendency  of  the  visual  axes  outward  (exophoria, 
insufficiency  of  the  internal  recti),  and  the  amount  of  the 
exophoria  is  represented  by  the  strength  of  prism  placed 

1  j  bh  $  fl&Sif**^***1* 

~T3   &- 


1 86       REFRACTION  AND  HOW  TO  REFRACT. 

base  inward  which  will  bring  one  light  directly  above  the 
other. 

To  Determine  Vertical  Insufficiency  (Hyperphoria). — 
Proceed  by  producing  lateral  diplopia.  Place  a  ten  centrad 
-j»  prism  base  inward  before  the  right  eye,  and  have  the  patient 
look  at  a  point  of  light,  as  in  testing  for  lateral  insufficiency. 
(It  is  always  well  to  have  the  point  of  light  just  in  front  of 
a  large  piece  of  black  felt  cloth  tacked  upon  the  wall.) 
,  If  the  two  lights  which  the  patient  sees  are  on  a  horizontal 
'I  line,  then  the  condition  is  presumably  one  of  equipoise.  But 
(if  the  right  light  is  lower  than  the  left,  there  is  a  tendency 
of  the  visual  axis  of  the  right  eye  to  be  higher  than  its 
fellow. )  As  to  which  muscle  is  at  fault,  this  test  will  not 
tell,  and  Stevens'  tropometer  will  have  to  be  used.  The 
amount  of  the  deviation  is  represented  by  the  strength  of 
prism  placed  base  down  before  the  right,  or  upward  before 
the  left  eye  which  will  bring  the  two  lights  into  a  horizontal 
line — /.  c.,  on  a  level. 

To  Determine  Lateral  Insufficiency  at  the  Reading 
Distance. — Have  the  patient  look  at  a  black  dot,  with  a 
black  line  two  or  three  inches  long  running  perpendicularly 
through  it,  at  a  distance  of  about  thirteen  inches.  This  is 
>  known  as  the  line-and-dot  test  of  von  Graefe,  and  on  a 
larger  scale  may  also  be  used  in  the  previous  tests.  A  prism 
of  seven  or  eight  centrads  is  placed,  with  its  base  down,  in 
front  of  the  right  eye.  If  the  patient  sees  two  dots  exactly 
one  above  the  other  on  one  line,  there  is  not  supposed  to  be 
any  insufficiency.  If,  however,  there  are  two  lines  and 
two  dots,  and  the  upper  dot  is  on  the  right,  there  is  in- 
sufficiency of  the  externi  (esophoria)  for  near.  The  amount 
of  the  insufficiency  is  represented  by  the  strength  of  prism, 
placed  base  outward,  before  the  left  eye  which  will  bring 
the  two  dots  exactly  on  one  line.  If  the  upper  dot  is  to 


MUSCLES. 

the  left,  then  there  is  insufficiency  of  the  intern!  (exophoria) 
for  near,  and  the  amount  of  the  insufficiency  is  represented 
by  the  strength  of  prism  placed  base  inward  over  the  left 
eye  which  will  bring  the  two  dots,  one  above  the  other,  on 
one  line. 

»  Another  method  for  testing  lateral  insufficiency  at  the 
reading  distance  of  1 3  inches  is  to  have  a  card  about  6 
inches  square,  and  on  this  card  to  draw  a  heavy  black  line 
about  3  inches  long;  this  line  is  to  be  horizontal.  At 
the  middle  of  the  horizontal  line  draw  a  heavy  black  line, 
one-half  an  inch  long,  extending  vertically  from  the  hori- 
zontal line  ;  this  short  vertical  line  to  be  capped  with  an 
arrow-point.  The  horizontal  line  is  divided  off  into  equal 


spaces,  each  3  l/$  millimeters  apart  and  .numbered  from  I 
to  1 5  each  side  of  the  arrow ;  those  to  the  left  of  the  arrow 
are  marked  "esophoria,"  and  those  to  the  right  of  the  arrow 
are  marked  "exophoria."  (See  Fig.  165.) 

To  use  this  method,  a  prism  of  8  centrads  is  placed  base 
down  before  the  right  eye  ;  this  doubles  the  scale  vertically ; 
the  upper  scale  belongs  to  the  right  eye.  The  number 
and  the  word  in  the  upper  scale  to  which  the  arrow  in  the 
lower  scale  points,  is  the  approximation  in  centrads  of  the 
amount  of  the  esophoria  or  exophoria.  For  instance,  if 
the  lower  arrow  points  to  figure  9  in  the  upper  scale  to  the 
right  of  the  upper  arrow,  that  is,  the  word  "exophoria," 


1 88         REFRACTION  AND  HOW  TO  REFRACT. 

then  there  is  approximately  9  degrees  of  "exophoria"  at 

this  distance  of  1 3   inches,  the  distance  at  which  this  scale 

is  intended  to  be  used. 

These  tests  for  insufficiencies  should  always  be  made  be- 
s.  fore  estimating  the  refraction,  and  also  after  the  correcting 

lenses  are  carefully  placed,  with  their  optic  centers,  before 

the  eyes. 

/    -To  avoid  confusion  in  making  these  tests,  when  a  point 
^  of  light  is  used  as  the  fixing  object,  it  is  customary  to  place 

a  piece  of  plane  dark  red  glass  before  one  eye,  so  that  the 


FIG.  166. — Maddox  Rod.  FIG.  167. 

>  |  red  light  always  corresponds  to  the  eye  with  the  red  glass. 
Or  a  Maddox  rod  (Fig.  166),  white  or  red,  may  be  used 
for  the  same  purpose.  This  may  be  a  series  of  rods  (see 
Fig.  167)  placed  in  a  metal  cell  of  the  trial -case,  and  the 
eye,  looking  through  it  at  the  light,  will  see  the  image  of 
the  flame  distorted  into  a  streak  of  broken  light.  A  strong 
-f-  cylinder  from  the  trial-case  will  answer  the  same  pur- 
pose. (As  the  rod  refracts  rays  of  light  opposite  to  its  axis, ; 
the  eye  will  see  a  streak  of  light  in  the  reverse  meridian  to 


MUSCLES.  189 

that  in  which  its  axis  is  placed.  To  expedite  the  deter- 
minations, the  rotary  prism  of  Cretes  or  the  revolving 
prisms  of  Risley  may  be  employed.  This  latter  apparatus 
(see  Fig.  168)  is  composed  of 
two  superimposed  prisms  of  I  5 
centrads  each,  and  mounted  in  a 
milled-edged  cell  of  the  size  of 
the  trial-lens.  By  means  of  a 
milled-edged  screw  these  prisms 
are  made  to  revolve  so  that  in 
the  position  of  zero  they  neutral- 
ize each  other,  and  when  rotated 
over  each  other  the  prism  j.1(.  l68 

strength  gradually  increases  un- 
til the  bases   of  the  prisms  come  together  and  equal    30 
centrads.     The  strength  of  the  prism  employed  is  indicated 
by  an  index  on  the  periphery  of  the  cell. 

Stevens'  Phorometer. — This  is  a  very  convenient  appa- 
ratus, composed  of  two  4-degree  prisms  placed  in  a  frame 
3  l/2  inches  from  the  eyes,  which  with  an  attached  lever  can 
be  rotated  so  as  to  test  the  strength  of  the  vertical  and 
lateral  muscles.  Indexes  and  letters  at  the  periphery  of 
the  frame  record  the  character  and  degree  of  the  insuffi- 
ciency. (See  Fig.  169.) 

Treatment  of  Insufficiencies. — As  ametropia  is  the 
most  common  cause  of  insufficiency,  the" first  consideration 
must  be  to  select  the  proper  correcting  glasses.  After  this 
has  been  accomplished,  if  the  insufficiency  still  persists  and 
the  patient  is  not  comfortable,  then  the  muscles  should  re- 
ceive careful  attention,  and  their  condition  be  studied  from 
every  point  of  view.  The  patient's  general  health  should 
be  looked  after,  and  if  at  all  defective,  must  have  remedies 
prescribed  for  its  improvement.  In  some  instances  the 


190 


REFRACTION  AND  HOW  TO  REFRACT. 


patient  may  have  to  give  up  any  close  application  of  the 
eyes  for  a  time  and  pursue  an  out-door  life.  Operative 
interference  (tenotomy)  must  not  be  entertained  until  all 
known  means  for  the  relief  of  the  muscular  asthenopia 
have  been  exhausted. 

The  prescribing  of  prisms,  as  a  fixed  rule,  for 
use,  which  correct  insufficiency,  except  in  vertical  erro/s,  is 
often  a  serious  mistake  on  the  part  of  the  surgeon,  as  in 
most  instances  they  often  do  more  harm  than  good  by  in- 
creasing the  difficulty.  Internally,  sedatives  will  frequently 


--  Phorometer 


Revolving 
Maddox  Rod 


Revolving 
Trial  Frame 
— Revolving 
Rotary  Prism 


FIG.  169. 

give  great  satisfaction  and  permanent  relief.  The  writer  is 
partial  to  the  use  of  bromids  with  small  doses  of  the  iodid 
of  potash  three  or  four  times  a  day.  The  modus  operand' 
is  not  clear.  The  only  guide  that  can  be  suggested  is  to 
use  sedative  treatment  and  rest  of  the  eyes  whenever  there 
is  a  congestion  of  the  choroid  and  retina  and  when  the 
ophthalmoscope  shows  the  nerve  edges  hazy,  the  retina 
woolly,  etc.  -  "In  another  class  of  patients  the  internal  use 
of  nux  vomica  is  the  treatment  par  excellence,  and  it  acts 
best  in  those  cases  where  the  nerve  edges  and  the  eye- 
ground  in  general  appear  clear  and  free  from  irritation. 


*s*r<? 


To  use  nux  vomica  it  must  be  given    in  the   form  of  the 
mcture   and   increased,  one   drop  at  each    dose,  until   the 
Datient  becomes  quite  tolerant  of  it,  taking  as  high  as  thirty, 
ty,  or  even    fifty  drops  three  times  a  da}',  and  then  the 
dfcse  is  gradually  diminished.      Xux  vomica  does  not  seem 
do  well  in  cases  in  which   the  bromids  are    indicated   as 

viceyersa.      (De  Schweinitz.) 

featment  of   Insufficiency  of  the   Internal   Recti. 
— Because  the  tests  for  heterophoria  at  6  meters  show  an  v 
ability  on  the  part  of  the  patient  to  maintain  equilibrium,  it  ^ 
must  not  be  supposed  that  there  may  not  be  an  insufficiency. 
The  normal  ratio  of  adduction  to  abduction  should  be  taken 
nto  consideration  in  every  instance  before  coming  to  any    . 
such  conclusion. 

i^After  the  proper  correcting  glasses  have  been  prescribed    ^ 
-jand  the  patient's  general  health  looked  after,  attention,  if  ^ 
necessary,  should  be   directed  to   strengthening   the  weak 
,muscles  ;  and  to  do  this  they  must  be  given  a  certain  amount 
"rnatic  exercise,  known  as  ocular  gymnastics.     That 
'success  shall  result  from  ocular  gymnastics  means  perse- 
verance on  the  part  of  the  patient  and  the  exercises  system- 
atically executed.     There  are  two  methods  of  procedure  :  in 
cases  of  exophoria — Dr.  George  M.  Gould,  "  Med.  Xews," 
-Nov.  18,  1893 — 

1.  Have  the  patient  "fix"  the  point  of  a  pencil,  or  the 
end  of  his  finger  held  at  arm's  length,  and  slowly  draw 
it  toward  the  bridge  of  the  nose.      If  diplopia  results  while 
doing  this,  the  exercise  should  cease,  and  be  repeated  from 

the  original   distance.      This,  is  a  very  convenient  exercise.,  v, 

rf»  <u  «*-IL<  ^*-*~+*^    Ikdv'Vf  rvyvon 
and  should  be  practised  several  times  a  day  and  a  number/ 

etot6*^**^£~t*ii^ »  y»*>  '>jL*fr.    f  \^^»^  '^"Wt-rtAJM*-     LA 

oftimes  at  each  sitting. 

2.  Prism  Exercises. — The  patient  is  placed,  standing     ^ 
about  a  foot  or  two  from  a  point  of  steady  light,  on  a  lev 

or  slightly  below  the  level  of  the  eyes,  and  told  to  look  at 


v 


> 


F92  REFRACTION     AND     HOW     *O     REFRACT. 

it., and   at  ^nothing  else.     In  this  position  a  pair  of  weak 
/  4ff  4  Ufa  fir  l 

prrsms  DasefouT,  in  a  trial-frame  are  placed  in  front  of  his 
eyes.  * 

Then  he  is  told  to  walk  slowly  backward  as  he  keeps 
his  eyes  fixed  on  the  point  of  light.  Should  diplopia  de- 
velop at  any  distance  short  of  20  feet,  then  he  is  to  raise 
the  prisms,  go  back  to  his  original  position,  and  start  over 
again.  Repeating  this  a  number  of  times  in  the  surgeon's 
office,  it  will  be  found,  in  most  instances,  that  at  the  first 
practice  a  pair  of  5  A  or  10  A  can  be  overcome  at  a  distance 
of  20  feet.  When  the  distance  of  20  feet  from  the  light  is 
reached  without  developing  diplopia,  the  patient  is  instructed 
to  slowly  count  20  or  30  (keeping  the  light  single  during 
this  time),  then  raise  the  prisms  (gazing  at  the  light),  and  to 
slowly  count  20  or  30  ag<yjy  ^This-exercise  is  repeated 
three  or  four  times  a  day^ma  a  number  of  times  at  each 
practice.  A  prescription  is  given  for  such  a  pair  of  square 
prisms  with  a  convenient  frame  to  wear  over  the  patient's 
glasses.  These  exercises  should,  as  a  rule,  be  conducted 
with  the  patient  wearing  his  correction.  Instead  of  the 
prism-frame,  the  patient  may  hold  the  square  prisms  with  his 
hands ;  but  these  are  tiresome  to  hold,  and  for  general  use 
the  prism-frame,  if  not  too  heavy,  is  preferable.  After  a 
few  days'  practice  at  home,  the  patient  returns,  and  stronger 
prisms  which  will  permit  the  patient  to  maintain  single 
vision  are  ordered.  This  practice  with  stronger  and  stronger 
prisms  is  repeated  until  the  patient  is  able  to  overcome 
prisms  greatly  in  excess  of  the  normal  ratio  of  adduction 
to  abduction.  It  is  often  well  to  develop  the  power  of  the 
internal  recti  to  three  or  four  times  the  strength  of  the  ex- 
ternal recti ;  for  when  the  exercises  are  stopped,  some  of  the 
strength  of  adduction  will  rapidly  disappear. 

It   has  been  incidentally  mentioned  that  prisms  should 


•  of  class  2-  ;ts  in  this  class, rs:  just  stat- 
,  are  usually  .     ; -epic  rrC  therefore  require 
to  corrections  .  ; restyopes  with  exophoria  hs 
v  reachec  a  stage  of  lifr  when  it  is  difficult 
ost  impossible  t©  p'_:t  new  life  irto  old  rt 


or 

ture 
ruscl 

ateneit.    aro    this   fart   becer.es  more    and   nr.ore    evider 
th<!   pt   passes   beyond   fifty   ;rc.    Developing   add 


and  it  c!eer.  seem  as  if  the  ar 

ike  the  cil  IT.  were  no  exception  to  this  st 


by  irpctisir.r  foxation  at  33  centimeters  -ray 

li  ch  a  gecc     i  in  some  ycur.r  ps,tut  * 


power 

-tion  with  fixation  or  iris 
is  in  Almost  every  instance  '-nd  with  few 
c  cations  a  waste  of  time  in  ps  after  fifty  fifty  f 
five  of  sixty  yrs.    These  ps  do  not  trke  kincHJ 
to  such  tr  and  in  the  Friters  experience ,pts  so  tr 
so jn  seek  assistance  elsewhere. 

In  gyperphoria  the  full  prismatic  correc 
tion( except  in  cases  of  presbyopia)  is  seldom  erdei 
ed-only  alout  two-thirds  of  it  and  this  divided  bet 
ween  the  two  eyes-base  down  in  one  and  b-se  u  befi 
re  the  othe  r . 

Testing  the  rusculsr  condition  at  "3  c 
centimeters  with  'he  presbyoric  near  correction  be 
fore  the  pt's  eyes, there  should  be  cbout  ten  degree 
s  of  exophoria  normally  at  this  di£:t&nce»but  if  fchi 
there  happens  to  be  I2,I4,or  16  degrees  ef  exophori 
then  the  presbyejsia  is  mnco~f  or  table  ard  complains 
correspomdl     when  uring  the  eyes  at  near  work  fi 

length  of  time.  The  treatment  of 

exophoria  at  the  working  cirtance  in  presbs*  is  to 
add  or  prescribe  prisms  bases  in  to  be  made  in  the 
near  correction.  The  amt  of  prisn  to  be  so  ordered 
is  usually  c-ivieed  between  the  two  eyes.  As  !•  de- 
grees of  exophoria  is  normal  for  this  distance  of 
^eteres  as  ^ust  rentioned, then  the  amount 
of         escribed  will  be  practically  two-thirds 
:he  amount  shown  is  excess  of  the  normal  it  1C 
^ees.        For  example., the  patient  at  fifty  yi 

'or  each  eye  plus  one  sphere  pe-riscopic 
and  has  a  vision  of  6/6  'n  each  eye  rith  this  cor- 
rection snd  does  not  reveal  any  insufficiency  at  6 
retere,but  when  p:us  2  sphere  is  added  for  near,*hi 
then  gt  33  cms  there  is  found  to  be  16  degrees  of 
exophoris. 


for    a    fe\v-  days    at 

r.e    teh   patient    returns    cor.p"!  sinin/g   that    a 
close  work   the    eyes   pain,feel    sore   to   th 
uch,has   occip  hdrche , smarting   of   lids    rnd 
urred   vision.        The   exophoria   in   this   inst 
"7  cgrees    in   excess   of   tfte    norr: 

amount.    Ordering   tv;o- thirds    of    this    amour J? 
ur  degrees)    divided   t etween   the    two    eyes, 
e    patient  rill    rec¥«ve   one   cf   the   following 
"  i  o  n  s ; 
.     .  .         . D.Ieriscopic 


11  so'  .     .  .  C.T  .  ' 

00  n  »•  t»  • 
•     • 


ase   in. 


f»r  near 


.    lus        .    .  ctr.l  -f  -  ed 


. 
l«rer 


rart 


for 

ef   the 


distance 
f«r 


'  -ed   2.:  D 


Base    i? 


o.s. 

near. 
.  .   i  u  c 
cer.ert  on 
near  the  fol: 

us  2 
he  Eair.e 
t if or  al i . 

exop 

"nethr  Degrees   of   e«op   at   6 

r-cterc   rith   the    sams   corert  ion(I  .OCCD  leriscop 
'n  each  eye)  it  would   not   be  wise   to   give 

ance    correction, but    to    allow  him   to   use   hi^ 
relative  .         and    at    the   car.e    time   to 

the  fixation  exers  if  he  becomes  unc 
e  in  the  use  *f  his  eyes  at  a  distan 
-.other  vrry  food  tr  in  esses  ©f  exop  to 
courage  convergence  is  to  prescribe  s  weak  so 


the 


•m 
ce 


en 

.on  of  eserin  or 
to  the  ounce ,  ST. 


'  lecarpin,  quarter  of 
use    ore   dfop    in   each 


a  gr 
eye   r 


MUSCLES.  193 

not  be  prescribed  in  combination  with  the  ametropic  cor- 
rection for  the  treatment  of  insufficiency,  and  yet  there  is 
an  occasional  exception  to  this  statement  in  cases  which 
must  have  prompt,  though  temporary,  relief,  Occasion- 
ally, the  relief  may  be  permanent  ;  but  this  will  not  hap- 
pen very  often.  When  ordering  prisms^  for  such  a  case,  it 
is  best  to  prescribe  them  in  the  form  of  hook  fronts,  so  that 
they  ma}-  be  thrown  aside  at  any  time.  In  hyperphoria  the 
full  prismatic  correction  (except  in  cases  of  presbyopia)  is 
seldom  ordered,  —  only  about  two-thirds  of  it,  and  this  is 
divided  between  the  two  eves  —  base  down  before  one,  and 


^m 

base  up  before  the  other.T^jT^  £ZZs£%  *%?*//*<***-  <f 

Treatment  of  Insufficiencyor  the  Extern!  (Esophoria). 
—  As  esophoria  is  a  tendency  of  the  visual  axes  to  deviate  in- 
ward,  it  will  be  found  that  patients  with  this  form  of  insuffi- 
ciency suffer  very  little,  if  at  all,  when  using  the  eyes  at  near 
work  ;  their  chief  discomfort  arises  from  using  the  eyes  for 
distant  vision.  The  "  shopping  headache,"  the  "  opera  head- 
ache," the  "  train  headache,"  may  be  due  to  this  form  of  in- 
sufficiency, but  it  is  not  so  apt  to  cause  discomfort  if  the 
ametropic  correction  is  worn  constantly.  In  other  words,  if  a 
hyperope  does  not  wear  his  distance  correction  and  accom-  /  y 
modates  at  the  same  time  that  he  endeavors  to  maintain  equi-  V  * 

poise  (relative  hyperopia),  he  may  at  times  suffer  severely.      ^/^/> 
If  the  symptoms  of  muscular  asthenopia  persist  after  pre-     fa  '  *+  "'     V 
scribing  the  ametropic   correction,  then  prisms,  bases  out,          ^•n  > 
may  be  prescribed  as  hook  fronts  to  be  worn  over  the  con- 
stant  correction  when  using  the  eyes  for  distance.     It   has 
been  the  writer's  experience   that  esophoria  of  two,  three, 
or  four  degrees  seldom  gives  the  possessor  any  discomfort 
whatever,  i  Prism   exercises  for  esophoria  give  very  little 
benefit,  and  are  often  a  waste  of  time  ;   yet  they  should  be       ^^T££ 
tried  thoroughly  if  the  case  appears  to  demand  it.  y  7 


194        REFRACTION  AND  HOW  TO  REFRACT. 

Treatment  of  the  Insufficiency  of  the  Superior  and 
Inferior  Recti. — Having  prescribed  the  ametropic  correc- 
tion, an  attempt  should  be  made  to  strengthen  the  weak 
muscles  by  prism  exercises — prism  base  down  before  one 
eye,  and  base  up  before  the  other  eye.  While  this  does  not 
;  often  give  satisfactory  results,  yet  it  should  be  tried  in  each 
instance.  If  prism  exercises  do  not  correct  the  difficult}-, 
then  prisms  which  overcome  most  of  the  insufficiency  should 
be  prescribed  for  constant  use.  Failing  in  this  second 
attempt  with  prisms  or  with  a  full  prismatic  correction,  then 
tenotomy  of  the  overacting  muscle  or  muscles  must  have 
consideration. 

Tenotomy. — As  previously  stated,  tenotomy  should  never 
be  resorted  to  until  every  other  known  means  of  relief  has 
been  tried,  and  even  then  no  hard-and-fast  rule  can  be 
given  for  the  amount  of  the  insufficiency  in  degrees  which 
will  prompt  such  a  procedure.  Some  patients  with  as 
much  as  four  or  six  degrees  of  esophoria  may  never  suffer 
the  least  annoyance ;  and  yet  other  patients  with  the  same 
amount  will  estimate  their  sufferings  as  almost  beyond  en- 
durance. And  the  same  statement  holds  good  in  other 
forms  of  insufficiency,  especially  exophoria.  The  question  of 
personal  equation,  the  patient's  nervous  system,  hysteric 
tendencies,  etc.,  must  all  be  considered  before  undertaking 
a  tenotomy  that  may  result  in  nothing  but  discourage- 
ment. 

If  an  operation  has  been  deemed  best,  then  it  is  for  the 
surgeon  to  decide  whether  he  will  divide  the  tendon  of  the 
strong  muscle  or  advance  the  weak  muscle,  or  both.  What- 
ever operation  or  operations  are  performed,  the  amount  of 
the  deviation  should  be  estimated  immediately  before,  as  well 
as  during  and  after,  the  operation.  When  a  simple  tenot- 
omy is  performed,  the  eye  is  usually  left  open  (unband- 


J  .    ^oo.    e^j^-^    '      '     ^ *— «•  I  I**  - 

Y.'hen   the   esop    ar.ts   to   6,8,«r   1C   Octrees,  then   the 

lent   rill    usually  have   narfte.^    s    such   as   ecul 
ar  hdacheE,^  runring  bfclrtwen   the   neck,& 

seme  tires    the    shoulders.  They   donot    have   am* 

cor.fort   when   read¥n'£A8'r8tTf'  flWg   and    in   fact    eft  en 
have   t©    abandon    any   prolonged  use   of   the    eyes   at 

'.    hear  work    .       "hen  the   patiert   h--s    several 
frees   of   esop    f   ~    :-i  stance  he   r.ust   use   his  din- 
ce    correct itn  which  usaally    corrects   any  furthe 
^ctmfort    in   the  use  of  /his   eyes     for  distance; 
but  he  may   continue    to  have   disc    at    any   near  work 
h  disc   as   ocular  pains, oc   hdaches, pains  running 
o   the  neck    end   sorretimes   frit   between   the    shoul 
s.  Such  pts   do   not   have   any   corcf   from   thier 

n   rding  or  rriting  or   any   close  work  vhich 
requires  the   eyes  to  nove    instead   of     chaining  fix 
For   instance    fa   sewing  werr.an  will    corr.e  with 
the    story   that    she   ca  n   sew  with  comfort   with  her 
glssces  on, but    that    she   cannot    read  with   cor.fort   & 
she    crnnot   undrrctand   why.      A    stenographer  who   rur 
the    typewriter  curing  the   day    suffers    from   the   s   s 
just   described  but    she   cm   sit    and   sew    in  the   ever 
ing   srd   not   get    a  hdache.          THe   trc-vble   lies   in  w 
v-eak    aBducting   i . ower.  The    treatment    is  net   to 

v;eal:en  but    t©  strenghten    abduction.  The  w    iter 

'  ethod  vhich  he   believes   to   be   o:  iginalis   to   pract 
aba   or   tu1 ning   outward   of  khz   each   eye   seprrat 
that    is   to   tuf-n   each   eye   outward  while   its  fell 
-s    covered.  The   pt    fixes  his  head   in   one   pos 

tion  j?.nd  not  to  turn  it  while  practiseng  ts  follow 
to  cover  the  eye  with  a  card  ?s  shown  and  in  such 
a  rranner  that  the  covered  eye^  cannot  see  what  th 
other  eye  is  io ing; then /ho Id ing  the  index  finger  p 
;oint  an  a  level  with  the  eye, the  finger  is  gradu- 
ally r.ad  eto  describe  a  Tusrter  circle  to  the  sair.c 

thre    e   being   exercised; the   eye   fixes   the 
point   of    the   finger   to    the        Unit   of   external 


rotation.  Fi?  st  one  eye  and  then  t 
minutes  after  each  meal.  Marked  in 
and  the  ?rr  ean  testify  to  remirkab] 
ghting  abduction.  Occasionally  t 
pt  rill  also  hav  eto  have  pri 

.After   partial    tenotomies   TI95   r 
after   a   tenotorr.y   if    any   annoying   ir 
ways  be    successfully  relievee 
rade    in    the   correcting  glasses. 

After  par   Ii   page  ?3I    add   - 

Cummar.  . 

The  f  ol  .owing  sice  is  given  ^s  the  \ 
tried  after  the  correction  is  si 
patient  sees  6/6  rith  the  f  •!!•?.  in£ 
is  not  to  be  ordered  until  the  var5 
this  coorection*-  to  mak  e  sure  that 
on. 

_/  0.2rsphere 

—  O.?r    sphere. 

-fC.^   ^cl;:   2txfchfi»5pm*xixiz.    *. 
-f"  O  I  ^      at    the   opposite    axis. 

—  ^,L^       st    the   same    axis 
^   0  ,  VT  *PP   axis 

crosser    cyl   lens 


/^fter   fecus    p??0   4th   line    ac 
their  use   is  of   advantage   to   pr 
^uire   lens   of  leesjff  .  v  .eight    anT    als« 
formerly   a   plusI5.COS.B.contine€   vri 
cyl    next    to   the   eye    and    the    convex 
and  «nweildy   lens,  so    that   nor  vith 
next   to    the   eye   and   the   outer  aurfac 
an    and   plus  8    in    the   hor.rr.er. 
with  very  lone:  laches    appreciate    to 
The    teric    Q!  so    allow   a  r.uch  neater 
cemented   on   to    the    concave    surface. 


•  ther  is  exercired  this  v  ay  for  5  or  ten 
ovrrert  rill  follow  this  tr  i:     ew  days 
results  by  this  very  si~  le  method  of  stren 
sctice    alone  will  not  suffuce  and  the 
s  (bases  in) 

t  is  an  interstir     -  rever  the  leas  that 

•^ems-ins,  it  can  Usually  but  not  al- 
cribing  the  necessary  correcting  prism  to  be 


ios  steps  in  refraction  one  the  lens  to  br 
d  to  be  eorrect.     For  ir.sta- nce,if  the 
lus  I. CO  cer.b  vrith  rlus  T.CC  cyl .  axisOO,  this 
s  lens  have  been  successively  placed  before 
he  eye  will  not  accept  some  other  coir.binsti- 


he  same  axis 


ic   1  ere ,-These   have   been  described( p54)but 
r^    and   cases   of   aphakia,as    these   cases   re- 
ens  which  will    enlarge   the   field   of  vision. 
llurr.Cl  at    axis    I6C   ras   mede  v:ith   the   2 

ras    placed   outward,  this  r.sde   a  very   thick 
e   toric   "lens   this  vrould   equal    a    plusT  .r. 
rould  have    a   plus    10   c;-l    in    the   vrrt.mefcidi- 
I  res   en0o;    toric   bee    of   enl. field.    I  eo-ple 

^  ere   than   the    old    cphero-r;  :r  lens, 

focrl    by   alloving   the    scale    e-   segment    to  be 
Vc  •       le   with   sen    Itive    retir!aa  do   no 


t  as  a  rule  ftnjty  t tries  as  they  per-  '  ' 
pheral  rays  to  fall  upon  j  ortions   of  the  ret 

which  do  not  tolerate  the  inc  eased  stirulatii 
n,  I  rest  as  a  rule  are  not  sir.ilarly 

disturbed. 

I age?94  add  .  Tused  bifocal s(Kryptok) *-This 
variety  is  also  known  as  "invisible" . It  is  not 
unlke  the  bifocal  shown  in  Fig. 185.  It  is  made 
by  taking  a  small  circular  piece  tf  flint  glass 
»nd  by  greet  heat  fusing  ®ne  surface  t»  a  crtwr 
glass-  piece;  the  cr«^n  glsss  is  square  in  she 
this  is  the  form  in  vhich  the  Krypttk  Sales  C« 
supply  the  opticians, who  then  grind  the  v 
curves  to  meet  the  conditions  of  the  oculist 
prescr: ptions. 

After  trifocals  :  age 295  add.    One-ptece  bifocl 
s.-This  is  rot  unlike  the  solid  or  ground  bifo- 
cals but  it  nor  ncde  in  the  toric  rurve  and 
neater  and  more  delicate  than  the  previaus  col- 
id  biffccal.  This  one-piece  bifocal  is  frequent- 
ly also  called"invisibletf .  Close  insp  I     Tle( 
ted  light  will  generally  sho^r  where  the  two  ed 
ge-s  come  together.    Its  think  they  will  av 
seeing  where  the  upper  and  lower  corr  come  to- 
gether, lut  they  will  see^/it  in  any  bifocal  arc 
the  term  invisible  applies  t®  the  friends  of  U 
petient  who  cannot  always  see  that  '.if teals  are 
being  wor  n.  latients  of  unknown  age  wearing 
bifocals  of  the  "inv"  variety  en . oy  the  fact 
that  their  friends  still  think  they  are  youngt' 


MUSCLES.  195 

aged)  so  that  visual  fixation  is  maintained,  and  the  muscle 
balance  tested  frequently  to  see  that,  by  subsequent  con- 
traction, the  insufficiency  does  not  return.  To  avoid  such  a 
misfortune  it  may  be  necessary  to  use  prism  exercises  dur- 
ing the  healing  process.  The  writer  is  not  an  advocate  of 
partial  tenotomies.  V"  *Z^-4Ji  'jfty™-  'Vt**'  •*  , 

Strabismus  (arpl<fta,  "  to  turn  as'ide  ")  ;  also  called  heter- 
otropia,  "  cross-eye  "  or  "  squint,"  or  manifest  squint.  This 
is  a  condition  of  the  eyes  in  which  the  amount  of  the 
insufficiency  is  so  great  that  it  can  not  (always)  be  over- 
come by  muscular  effort ;  and,  in  fact,  inspection  often  shows 
the  manifest  condition.  Or  strabismus  may  be  defined  as 
the  condition  in  which  the  visual  axis  of  jone  eye  is  deviated 
from  the  point  of  fixation.  The  eye  which  has  the  image 
of  the  object  on  its  fovea  is  spoken  of  as  the  fixing  eye, 
while  the  other  eye  is  termed  the  squinting  or  deviating 
eye.  The  squinting  eye  does  not  always  have  normal 
visual  acuity ;  and,  in  fact,  correcting  lenses  will  not 
always  produce  such  a  result. 

Varieties  of  Strabismus.  —  Convergent,  divergent, 
vertical,  monolateral,  alternating,  periodic,  concomitant, 
and  paralytic. 

Convergent  squint  (con,  "together,"  and  vergere,  "to 
incline  or  approach  ")  ;  also  called  internal  squint  (strabis- 
mus convergens),  esotropia.  This  is  the  condition  in  which 
the  visual  axis  of  one  eye  is  deviated  inward,  the  other 
fixing  the  object  ;  or  one  eye  fixing  an  object,  the  visual 
axis  of  the  other  eye  crosses  that  of  the  fixing  eye  closer 
than  the  object.  (See  Fig.  163.)  This  is  thePmost  common 
form  of  squint.  Both  eyes  have  some  form  of  hyperopia, 
as  a  rule,  the  squinting  eye  usually  being  the  most  ame- 
tropic.  The  diplopia  as  a  result  of  this  condition  is 

X 

homonymous. 


196        REFRACTION  AND  HOW  TO  REFRACT. 

Divergent  squint  (di,  ''apart,"  and  vergere,  "to  in- 
cline ")  ;  also  called  external  squint  (strabismus  divergens), 
exotropia.  (See  Fig.  164.)  This  is  the  condition  in  which 
the  direction  of  the  visual  axis  of  one  eye  is  directed  out- 
ward, the  other  eye  fixing  the  object ;  or  one  eye  fixing 
an  object,  the  visual  axis  of  the  other  eye  can  cross  it  only 
by  being  projected  backward.  JYhe  diverging  eye  is  usually 
myopic. 

Monolateral  (one-sided)  squint ;  also  called  constant. 
It  may  be  either  convergent  or  divergent,  but  the  squint  is 
a  constant  condition  of  one  eye. 

Alternating  Squint. — This  is  the  condition  in  which 
at  different  times  the  right  eye  fixes  and  the  left  eye  squints, 
or  the  left  eye  fixes  and  the  right  eye  squints.  The  vision 
in  one  eye  may  be  as  good  as  that  of  its  fellow. 

Periodic  squint ;  also  called  intermittent.  This  is  the 
condition  in  which  the  visual  axis  of  one  eye  occasionally 
deviates.  It  may  eventually  become  constant,  and  is  often 
the  first  indication  of  a  beginning  convergent  or  divergent 
squint. 

Vertical  Squint. — This  is  the  condition  in  which  the 
visual  axis  of  one  eye  is  deviated  upward.  Also  called 
hypertropia. 

Concomitant  Squint. — This  is  the  condition  in  which 
the  squinting  eye  has  freedom  of  movement  and  will  follow 
its  fellow,  and  yet  one  eye  deviates  (inward  or  outward) 
because  of  an  inability  to  "  fix." 

Paralytic  Squint. — This  is  the  opposite  condition  from 
concomitant,  in  which  there  is  a  restriction  in  the  move- 
ment of  one  eye  in  a  certain  direction,  due  to  a  palsy  of 
one  or  more  of  the  muscles. 

Causes  of  Squint. — These  are  many  and  various.  The 
chief  causes,  however,  are  :  (i)  Ametropia,  which  may  pro- 


MUSCLES.  1 97 

duce  a  change  in  the  normal  relationship  between  accom- 
modation and  convergence  ;  (2)  anatomic  anomalies  ;  (3) 
mechanic  anomalies  ;  and  (4)  amblyopia. 

I.  Ametropia  produces  a  change  in  the  normal  relation- 
ship between  accommodation  and  convergence.  While  it  is 
possible  for  accommodation  to  take  place  without  conver- 
gence, or  convergence  without  accommodation,  yet  there  is 
an  affinity  between  the  two  processes  which,  if  materially  in- 
terfered with,  will  produce  diplopia  and  eventually  squint.  In 
speaking  of  relative  hyperopia,  it  was  shown  that  the  accom- 
modative effort  was  accompanied  by  contraction  of  the  inter- 
nal recti  muscles  (convergence) ;  so  that  in  hyperopia  of,  say, 
four  diopters,  accommodating  for  infinity  convergence  would 
be  stimulated  to  a  proportionate  degree  at  the  same  time  ;, 
and  if  accommodating  for  a  near  point,  the  hyperope  must 
accommodate  and  converge  just  that  much  more.  The 
result  is  that  a  person  with  a  hyperopia  of  any  considerable 
amount  frequently  squints  inward  in  the  effort  to  maintain 
! binocular  vision.  If,  now,  one  eye  is  more  hyperopic  than 
the  other,  the  difficulty  of  adjusting  convergence  to  accom- 
modation is  increased.  Say  that  the  right  eye  has  3 
diopters  and  the  left  4  diopters  of  hyperopia  ;  then  the  two 
eyes  each  exert  6  diopters  to  fix  at  1 3  inches  ;  the  left  eye 
still  has  i  diopter  of  its  hyperopia  remaining,  and  with  the 
result  that  the  retinal  image  of  that  eye  is  not  clear,  and 
accommodation  is  still  further  taxed,  stimulating  at  the 
same  time  the  internal  rectus,  so  that  the  left  eye  deviates 
inward  and  ultimately  remains  convergent.  This  act  of 
convergence  explains  the  presence  of  convergent  squint  in 
hyperopia,  and  also  shows  why  the  squinting  eye  usually 
has  the  higher  refractive  error.  It  must  not  be  supposed 
that  all  hyperopic  eyes  have  a  squint,  as  some  of  these  can 


198        REFRACTION  AND  HOW  TO  REFRACT. 

accommodate  without  converging  in  a  proportionate  de- 
gree, and  this  is  especially  so  when  the  amount  of  the 
hyperopia  is  the  same  in  both  eyes. 

^  Myopic  eyes,  in  contradistinction  to  hyperopic  eyes,  can 
not  accommodate  beyond  their  far  points,  but  must  con- 
verge. If  the  myopia  is  8  diopters,  then  these  eyes  would 
have  to  converge  8  meter  angles  to  fix  an  object  at  that 
distance  (5  inches)  without  any  accommodative  effort.  It 
must  also  be  borne  in  mind  that  myopic  eyes  are  long  eyes, 
and  that  to  converge  8  meter  angles  means  a  great  effort 
on  the  part  of  the  internal  recti  muscles,  and  this  force  can 
not  be  continued  for  any  length  of  time  without  discomfort ; 
the  result  is,  convergence  is  relaxed,  and,  one  eye  remaining 
fixed,  the  other  is  turned  outward.  This  is  much  more 
likely  to  happen  if  one  eye  is  more  myopic  than  the  other. 
This  explains  the  presence  of  divergent  squint  in  cases  of 
myopia.  But  it  must  not  be  supposed  that  all  myopic  eyes 
necessarily  have  squint,  as  some  of  them  have  roomy  orbits, 
strong  internal  recti  muscles,  and  a  short  interpupillary 
distance. 

2.  Anatomic  Anomalies. — This  applies  especially  to  the 
breadth  of  the  face  (skull)   and  the  size   of  the  eye  and 
orbit.     The  broad  face,  which  naturally  gives  a  long  inter- 
pupillary  distance,  predisposes  to  greater  convergence  than 
the  narrow  face.     The  long,  myopic  eye  would  not  have 
the  freedom  of  movement  that  the  short  eye  possesses  in 
the  same-sized  orbit. 

3.  Mechanic  Anomalies. — This  refers  especially  to  the 
length    and  strength  of  the  extraocular  muscles.     Short 
and  strong  internal  recti  would  predispose  to  convergent 
squint,  whereas  strong  external  recti  would  develop  diver- 
gent squint. 


MUSCLES.  199 

4.  Amblyopia. — Statistics  show  that  from  thirty  to 
seventy  per  cent,  of  all  squinting  eyes  are  amblyopic.  The 
cause  of  the  amblyopia  may  be  that  the  eye  was  born 
defective  in  its  seeing  quality — /.  e.,  the  cones  at  the  fovea, 
the  optic  nerve,  or  the  visual  centers  in  the  brain  may  be  at 
fault.  Or  if  born  perfect  and  having  its  visual  axis  deviated 
by  one  of  the  many  causes  above  mentioned,  it  may  be- 
come amblyopic  from  not  being  used  (amblyopia  exanpp- 
sia).  This  consideration  of  cause  and  effect  is  most  impor- 
tant from  a  prognostic  point  of  view. 

Among  other  causes  of  squint  must  be  mentioned  opaci- 
ties of  the  media,  as  nebula  of  the  cornea,  or  any  want  of 
transparency  in  the  cornea  at  or  near  the  visual  axis,  or 
polar  or  nuclear  cataract.  Temporary  or  intermittent 
squint  may  result  from  vitreous  opacities,  or  from  the  rem- 
nant of  a  hyaloid  artery  passing  in  front  of  the  fovea.  Parents 
occasionally  delude  themselves  with  the  idea  that  the 
child's  squint  is  the  result  of  whooping-cough,  measles, 
teething,  sucking  the  thumb,  or  imitating  a  companion, 
etc.,  and  are  slow  to  believe  that  there  can  be  any  refrac- 
tive error,  forgetting  that  the  supposed  causes  they  men- 
tion may  be  but  coincidences. 

To  Estimate  the  Amount  of  the  Strabismus  or 
Squint. — This  is  not  always  easy  at  the  beginning  of  the 
examination,  for  the  reason  that  the  squinting  eye  has  long 
since  learned  to  ignore  the  false  object ;  and  if  the  angle  of 
the  strabismus  is  large,  the  surgeon  will  have  to  reduce  it  in 
part  with  a  prism,  so  that  the  patient  can  see  the  false  object ; 
and  if  this  is  a  point  of  light,  a  piece  of  dark  red  glass  will 
have  to  be  placed  in  front  of  the  fixing  eye.  The  strength 
lof  the  prism  required  to  bring  the  two  lights  together  will 
/  I  be  the  prismatic  estimate  of  the  deviation.  Or  the  amount 
of  the  squint  may  be  roughly  determined  with  the  strabis- 


2OO 


REFRACTION  AND  MOW  TO  REFRACT. 


mometer.  (See  Fig.  170.)  This  is  a  piece  of  bone  or  ivory 
hollowed  on  one  side  so  as  to  fit  the  curve  of  the  eyeball. 
Its  edge  is  graduated  in  millimeters.  This  device  is  held 
gently  against  the  lower  lid  of  the  squinting  eye,  so  that 
the  zero  (o)  mark  corresponds  to  the  center  of  the  pupil  as 
the  eye  fixes  a  distant  object,  the  fellow-eye  being  under 
cover.  When  the  cover  is  removed, 
the  squinting  eye  again  deviates, 
and  the  amount  of  the  deviation  is 
again  noted  by  the  position  of  the 
center  of  the  pupil  of  the  squinting 
eye  over  the  millimeter  line  on  the 
instrument.  Each  millimeter  of 
j  deviation  is  supposed  to  represent 
j  5  degrees  of  deviation.  This  device 
is  not  reliable,  and  is  not  in  common 
use. 

A  more  reliable  estimate  is  ob- 
tained by  measuring  the  deviation 
on  the  arc  of  the  perimeter.  (See 
Fig.  171.)  To  do  this,  the  patient 
is  seated  with  the  squinting  eye 
FlG  I70  opposite  to  the  fixation  point  (R) 

and  instructed  to  look  at  a  distant 

/object  (R)  across  the  room,  so  that  the  object,  the  fixation 
point,  and  the  squinting  eye  (R)  are  in  line  ;  this  line  repre- 
sents the  direction  which  the  eye  would  take  normally. 
The  observer,  taking  a  lighted  candle,  places  it  at  the  fixa- 
tion point  and  gradually  moves  it  outward  along  the  inner 
surface  of  the  arc  until  his  own  eye,  directly  back  of  the 
flame,  sees  an  image  of  the  flame  at  the  center  of  the  pupil 
of  the  squinting  eye.  The  degree  mark  on  the  arc  from 
which  the  flame  was  pictured  represents  the  amount  of  the 


MUSCLES. 


2O  I 


deviation  or  angle  of  the  strabismus ;  this  angle  being 
formed  by  the  visual  axis  with  the  direction  of  the  normal 
visual  line.  The  degree  mark  on  the  arc  is  in  front  of  the 


optic  axis  and  not  the  visual  axis,  but  for  purposes  of 
approximation  they  are  considered  as  the  same. 

Treatment  of  Strabismus. — As  ametropia  is  the  chief 
factor  in  the  cause  of  squint,  this  cause  must  be  promptly 


2O2        REFRACTION  AND  HOW  TO  REFRACT. 

removed  by  the  use  of  correcting  glasses.     The  correction 
of  the  ametropia  means  four  essentials  :    ' 

1.  In  young  subjects  the  eyes  must  be  put  at  rest,  and 
kept   at  rest  for   two,  three,  or  four  weeks,  with  a  reliable 
cycloplegic    and    dark     glasses.       Preference    is    given    to 
atropin  in  each  instance,  the  writer  considering  it  folly  to 
use  homatropin'  in  such  cases. 

2.  During  the  use  of  the  cycloplegic,  the  lenses  which 
correct  the  ametropia  are  selected  with  care  and  the  greatest 
precision,  by   every  known   means    to    this  end  ;   and  just 
here  is   the  place  of  all  places  to  use  the   retinoscope,  as 
most  cases  of  strabismus  appear  in  children,  and,  too,  the 
squinting  eye  often  being  amblyopic,  can  not  assist  in  the 
selection  of  the  glass. 

3.  The   correcting  glasses  are  ordered  in  the  form  of 
spectacles,  and  are  to  be  worn  from  the  time  of  rising  until 
going  to  bed.     The  strength  of  the  glasses  should  be  as 
near  the  full  correction  as  it  is  possible  to  give. 

4.  The  "  drops  "  are  continued  for  a  day  or  two  after 
the  glasses  have  been  obtained,  and  in  this  way,  while  the 
drops  are  still  in  the  eyes,  and  as  their  effect  slowly  wears 
away,  the  eyes  gradually  become  accustomed  to  the  new  or 
natural  order  of  accommodation  and  convergence.     After 
the  cycloplegic  has  entirely  disappeared,  the  patient  should 
be  carefully  restricted  in  the  use  of  the  eyes  for  near-work 
for  several  days  or  weeks.  j 

As  hyperopia  and  astigmatism  in  combination  are  gener- 
ally congenital  conditions,  it  therefore  follows  that  conver- 
gent squint  appears  quite  early  in  life,  as  soon  as  the  child 
begins  to  concentrate  its  vision  on  near  objects.  The 
squint,  at  first  periodic  or  intermittent,  finally  becomes  con- 
stant. Such  eyes  should  be  refracted  at  once,  and  before 
amblyopia  exanopsia  can  be  established.  It  is  interesting 


„    i 

NS 


MUSCLES. 

to  note  that  the  eyes  in  many  young  children  begin  to  fix' 
or  lose  their  squint  as  soon  as  cycloplegia  is  established. 
The  prognosis  is  favorable  for  good  vision  with  glasses 
when  this  occurs.  It  will  also  be  observed  in  other  sub- 
jects that  while  the  drops  are  in  the  eyes  and  glasses, 
worn  constantly,  the  squint  disappears  entirely  ;  but  as  soon 
as  the  cycloplegia  passes  away  and  near  vision  is  attempted, 
the  squint  returns,  and  vision  falls  back  in  the  squinting 
eye  to  almost  the  same  point  that  it  had  before  the  cyclo- 
plegia. This  occurs  in  cases  where  the  amblyopia  is  becom- 
ing established,  or  where  there  is  a  strong  muscle  devi- 
ating the  eye.  If  the  squint  is  due  to  amblyopia  exanopsia, 
then  the  vision  may  be  improved  in  one  of  two  ways,  as 


FIG.  172. 

suggested  by  Dr.  G.  M.  Gould,  "  Amblyopiatrics,"  "  Med. 
News,"  Dec.  31,  1892.  One  way  is  to  use  drops  in  the 
fixing  eye,  and  thus  compel  the  squinting  eye  to  do  the 
seeing  ;  or  the  other  way  is  to  cover  the  fixing  eye  with  a 
blank  over  the  glass  (see  Fig.  172),  and  have  the  patient 
practise  in  this  way  for  one  or  tw/o  hours  each  da^y  using 
the  squinting  eye  alone^  *£^~  ^^ 

Worth's  Amblyoscope  or  "  Fusion  Tubes.1" — To 
cultivate  or  develop  binocular  vision  Worth  has  given  us  an 
instrument  which  he  calls  an  amblyoscope.  (See  Fig.  173.) 
This  instrument  consists  of  two  halves  joined  by  a  hinge. 
Each  half  consists  of  a  short  tube  joined  to  a  longer  one 
at  an  angle  of  120  degrees;  at  the  junction  of  the  tubes  is 


2O4        REFRACTION  AND  HOW  TO  REFRACT. 

an  oval  mirror.  A  translucent  glass  object  slide  is  placed 
at  the  distal  end  of  each  tube.  At  the  hinged  ends  are 
lenses  whose  focal  length  equals  the  distance  of  the  re- 
flected image  of  the  object  slide  ;  in  front  of  these  lenses  are 
grooves  into  which  additional  lenses  of  the  trial  case  may 
be  placed  to  correct  the  refractive  error  of  the  patient.  The 
two  halves  of  the  instrument  are  united  by  an  arc,  having  a 
long  slot  at  one  end  and  an  adjusting  screw  at  the  other. 
The  object  slides  can  be  brought  together  to  suit  a 
convergence  of  60  degrees,  or  a  divergence  of  the  visual 
axes  of  30  degrees.  When  the  adjusting  screw  is  used  an 
additional  movement  of  10  degrees  is  obtained. 


FIG.  173. — Worth's  Amblyoscope.      (Reduced  size. ) 

At  the  far  end  of  each  tube  there  is  also  a  square  slot 
into  each  of  which  may  be  placed  half  a  pictured  object; 
for  instance,  a  picture  of  the  right  side  of  a  man,  showing 
his  arm  and  leg  extended,  may  be  placed  in  the  left  tube, 
and  in  the  right  tube  is  placed  a  picture  of  the  same  size, 
of  the  left  side  of  the  man  with  his  leg  and  arm  similarly 
extended.  When  the  patient  looks  into  the  tubes,  the  sur- 
geon (or  the  patient)  may  adjust  the  tubes  until  the  two 
half  pictures  unite  and  form  one  complete  picture.  Or  the 
picture  in  one  tube  may  be  a  picture  frame,  and  in  the  other 
tube  is  a  picture  of  an  animal  or  an  object,  the  idea  being 


MUSCLES.  2O5 

to  have  the  patient  so  fuse  the  two  pictures  that  the  object 
is  placed  in  the  frame.  There  are  many  different  pictures 
accompanying  the  instrument  so  as  to  give  variety  to  the 
daily  exercises  and  thus  maintain  the  patient's  interest. 
This  instrument  is  certainly  a  valuable  one  and  in  many 
instances  accomplishes  its  purpose. 

Cases  that  are  cured  by  correcting  the  ametropia  must 
wear  their  glasses  constantly.  Glasses  in  such  cases  can 
seldom  be  abandoned.  In  young  children  the  squint  re- 
turns almost  at  the  instant  the  glasses  are  removed.  The 
earliest  age  at  which  glasses  can  be  prescribed  is  three^ 
years  or  thereabouts,  as  it  would  be  unreasonable  in  most 
cases  to  expect  a  child  to  appreciate  the  glasses  as  anything 
but  a  toy  before  this  age. 

The  younger  the  patient  when  glasses  are  prescribed,  the 
more  favorable  the  prognosis  and  less  likelihood  of  a  ten- 
otomy.  The  older  the  patient  when  glasses  are  ordered, 
the  less  the  likelihood  that  glasses  will  cure  the  squint  and 
the  greater  probability  of  a  tenotomy  being  necessary. 
This  is  explained  from  the  fact  that  the  squint  having  per- 
sisted for  a  long  time,  the  muscle  which  held  the  eye  in  the 
deviated  position  has  grown  strong  and  the  opposing  mus- 
cle weak. 

The  correction  of  squint  by  glasses  applies  particularly 
to  cases  of  the  concomitant  (convergent  or  divergent)  form. 
Vertical  squint  is* seldom  cured  by  correcting  glasses  alone. 
Prisms  will  occasionally  substitute  for  an  operation. 

Monocular  and  alternating  squint  are  greatly  relieved 
by  the  correction  of  the  ametropia,  and  may  or  may  not 
be  cured  with  glasses  alone. 

Periodic  or  intermittent  squint,  if  due  to  permanent  opaci- 
ties in  the  media,  can  not,  as  a  rule,  be  cured  by  any  form 
of  treatment. 


2O6        REFRACTION  AND  HOW  TO  REFRACT. 

Paralytic  squint  is  not  a  part  of  the  subject-matter  of  this 
work.  Cases  of  concomitant  squint  are  generally  amenable 
to  operative  treatment,  whereas  cases  of  paralytic  squint  are 
not. 

It  may  be  stated  as  a  good  rule  to  follow  that  no  case 
should  ever  be  operated  upon  until  the  glasses  u'Jiicli  cor- 
rect tlic  ametropia  have  been  worn  constantly  for  several 
weeks  after  all  apparent  improvement  has  ceased.  If  cases 
for  operation  can  be  selected,  the  best  age  is  about  puberty, 
when  the  muscles  have  reached  a  fair  state  of  development. 
If  the  squint  is  due  to  an  anatomically  short  muscle,  then 
there  need  not  be  any  great  delay  in  operating  after  glasses 
have  been  ordered. 

Whenever  a  tenotomy  has  been  performed,  the  eyes 
should  again  be  carefully  refracted,  as  it  is  a  well-estab- 
lished fact  that  tenotomy  often  relieves  a  tension  that  will 
materially  change  the  radius  of  corneal  curvature ;  and 
hence  the  amount  of  the  astigmatism  and  the  cylinder  axis 
will  be  altered. 

Tenotomy. — For  convergent  squint,  if  of  moderate 
degree,  division  of  the  tendon  of  the  internus  of  the  con- 
verging eye  may  be  sufficient ;  but  if  the  squint  is  consider- 
able, the  tendons  of  both  interni  may  have  to  be  divided. 
Occasionally,  it  is  necessary  to  divide  the  internus  and 
advance  the  externus. 

For  divergent  squint,  if  of  moderate  degree,  division  of 
the  tendon  of  the  externus  of  the  diverging  eye  may  be 
sufficient ;  but  if  the  squint  is  considerable,  the  tendons  of 
both  externi  may  have  to  be  divided.  Occasionally,  it  is 
necessary  to  divide  the  externus  and  advance  the  internus. 

For  vertical  squint,  tenotomy  of  the  stronger  superior  or 
stronger  inferior  rectus,  or  both,  may  be  necessary. 

It  is  good  practice  in  every  instance,  before  "  rushing  " 


MUSCLES.  2O7 

into  an  operation  for  squint,  to  take  the  field  of  vision  and 
search  carefully  for  a  central  scotoma,  which,  if  present, 
should  put  the  surgeon  on  his  guard  against  operative 
interference  with  the  hope  of  obtaining  any  result  other 
than  cosmetic  ;  and  even  then  there  is  grave  danger  that 
the  case  will  soon  lapse  into  the  former  state  of  deviation, 
or  possibly  deviate  in  the  opposite  direction. 


CHAPTER  VIII. 

CYCLOPLEGICS.—  CYCLOPLEGIA.—ASTHEN- 
OPIA.—  EXAMINATION  OF  THE  EYES. 


A  cycloplegic  (from  the  Greek,  y.u*h>s,  "a  circle,"  —  /.  c., 
the  ciliary  ring,  —  and  ^tyry,  "  a  stroke  ")  is  a  drug  which 
will  temporarily  paralyze  the  action  of  the  ciliary  muscle. 

A  mydriatic  (from  the  Greek,  nuSptaats,  "  enlargement 
of  the  pupil  ")  is  a  drug  which  will  temporarily  dilate  the 
pupil. 

Atropin  will  dilate  the  pupil  and  also  cause  a  paralysis 
of  the  ciliary  muscle.  Cocain  will  cause  a  dilatation  of  the 
pupil,  but  will  not  paralyze  the  action  of  the  ciliary  muscle. 
A  cycloplegic  is  also  a  mydriatic,  but  a  mydriatic  is  riot 
necessarily  a  cycloplegic. 

The  Uses  of  a  Cycloplegic.  —  (i)  To  temporarily  sus- 
pend the  action  of  the  ciliary  muscle,  or  to  put  the  eye  in 
such  a  state  of  rest  that  all  accommodative  effort  is  for  a 
time  suspended  while  the  static  refraction  is  being  estimated. 
(2)  The  retina  and  choroid  are  given  an  opportunity  to 
recover  from  irritation  and  congestion  incident  to  eye-strain 
("  eye-stretching  ").  There  are  many  different  cycloplegics 
employed  for  estimating  the  static  refraction,  and  each  has 
particular  qualifications  for  individual  cases.  Cycloplegics 
may  be  classed  as  of  three  kinds  :  (i)  those  the  effect  of 
which  passes  away  slowly  ;  (2)  those  the  effect  of  which 
passes  away  moderately  fast;  and  (3)  those  the  effect  of 
which  is  very  brief. 

:/The   first  effect  of  a  cycloplegic  is  its  mydriatic  quality, 

208 


>  after  which  the  accommodative  effort  is  suspended.  The 
paralysis  is  not  permanent.  The  following  table,  from  Jack- 
son, shows  the  length  of  time  paralysis  persists  and  the 
time  it  takes  for  the  ciliary  muscle  to  fully  recover  : 

J^Atropirt,  effect  begins  to  dimi 

Daturin,  '  '  3   " 

3  " 

2    " 

12  hours. 


Hyoscyamin, 
Duboisin, 
Scopolamin, 
•7  Homatropin, 


ish  in    4  days  ;  complete  recovery,  15  days»> 


lo 
8 
8 
6 

2 


>\ 


If  a  solution  of  one  of  the  above-mentioned  cycloplegics  be 
instilled  into  the  conjunctival  sac  of  a  healthy  eye,  it  will  be 
carried  by  the  blood-  and  lymph-vessels  at  the  sclerocorneal 
junction  into  the  ciliary  muscle  and  iris,  where  it  acts 
directly  upon  the  nerves  and  ganglia  of  these  structures, 
and  the  aqueous  humor  also  receives  some  portion  of  the 
.drug.  If  cautiously  used,  the  action  will  be  limited  to  one 
I  eye,  showing  that  the  drug  does  not  pass  through  the  car- 
diac circulation  ;  otherwise,  the  pupil  and  ciliary  muscle  of 
the  fellow-eye  would  be  similarly  affected. 

Some  conjunctivas  are  very  sensitive  to  any  of  these 
drugs,  and  develop  an  inflammation  so  severe  in  individual 
instances  as  to  resemble  ivy  poisoning  of  the  lids.  f  Duboisin 
especially,  and  hyoscyamin,  by  absorption,  may  develop 
hallucinations  and  even  a  loss  of  coordination./ 

Any  cycloplegic,  in  fact,  when  carelessly  used,  may  pro- 
duce very  unpleasant  symptoms,  such  as  dizziness,  dry 
throat,  flushed  face  and  body  (mistaken  for  scarlatina),  rapid 
pulse,  a  slight  rise  of  temperature,  and  delirium.  To  avoid 
such  an  annoyance,  which  is  apt  to  reflect  discredit  upon 
the  physician  and  upon  the  profession  in  general,  the  patient 
should  always  be  given  definite  instructions  how  to  use  the 
drug  in  each  instance.  Stopping  the  use  of  the  drug  and 
18 


2IO       REFRACTION  AND  HOW  TO  REFRACT, 

applying  cold  compresses  will  relieve  the  conjunctivitis,  and 
if  constitutional  symptoms  manifest  themselves,  a  dose  of 
paregoric,  cooling  drinks,  a  darkened  room,  and  stoppin 

/the  use  of  the  ,drug  will  soon  restore  the  patie 
~CSy  fc^-v 

,  FORM  OF 

Name,  MR.  BROWN. 
K.     Atropin.  sulphatis,      ...........    gr.  j 

Aquae  dest.,     ..............  ^S1}'  ' 

M.     Ft.  sol.     Label,  poison  drops  !  ! 

SlG.  —  One  drop  in  each  eye  three  times  a  day,  as  directed. 

R.     Dropper.  DR. 

Date,  Tuesday,  March  14,  1899. 

« 


The  reason  for  labeling  this  prescription  "poison  drops" 
is  not  to  frighten  the  patient,  but  to  caution  him  against 
leaving  the  medicine  where  children  may  get  hold  of  it, 
and  at  the  same  time  to  let  him  understand  that  it  is  to  be 
used  and  handled  with  care. 

Mr.  Brown  is  told  to  have  one  drop  put.  in  each  eye 

'  jthree  times  a  day,  after  meals,  and  to  report  at  the  office  on 

Thursday  (the  prescription  is  given  on  Tuesday  in  this  case). 

The  reason  for  using  the  drug  for  this  length  of  time  is  to 

)?  insure  complete  paralysis,  and  also  to  give  the  eyes  a  physi- 
ologic rest.  In  having  these  "  drops"  put  in  the  eyes,  the 
patient  should  tip  his  head  backward  and  turn  his  eyes  down- 
ward, and  as  the  upper  lid  is  drawn  up,  one  drop  (from  the 

•  dropper)  is  placed  '(not  dropped)  on  the  sclera  at  the  upper  and 
outer  part.  After  the  drops  are  placed  in  the  eyes,  as  far  away 
s  from  the  puncta  lachrymalia  as  possible,  the  patient  holds 
the  canaliculi  closed  by  gently  pressing  with  the  ends  of  the 
index  fingers  on  the  sides  of  the  nose  at  the  inner  canthi 
for  a  minute  or  two.  If  more  than  one  drop  enters  the  eye, 

/  it  will  run  over  on  to  the  cheek,  and  should  be  wiped  off. 
With  children,  these  instructions  are  not  so  easy  of  exe- 


CYCLOPLEGICS.  2  1  I 

cution,  and  the  writer  has  seen  a  few  such  clinical  subjects 
flushed  and  delirious  from  gross  carelessness  on  the  part  of 
parents  in  dropping  the  medicine  into  the  inner  canthi, 
where  it  soon  passed  into  the  nose,  or  else  the  drug  is 
allowed  to  flow  over  the  cheek  and  into  the  child's  open 
mouth. }  Ordinarily,  there  need  never  be  any  discomfort 
from  the  use  of  these  drugs  beyond  a  slight  dryness  of  the 
fauces. 

Caution. — Cycloplegics    should    never   be    used    when 

•  •  ~~^  Jp*' 

there  is  the  least  suspicion   of  glaucoma  in   one  or  both  (j/y£' 

eyes.       Cycloplegics    should    not    be    used    in    the    eyes 
of  nursing  women;    such  patients   are   peculiarly   suscep-"' 
tible  to    the  action    of    these    drugs,    and  the    mammary 
secretion  may  thereby  be  diminished  in  amount.     After  the 
./  age  of  forty-five  or  fifty  years,  or  in    the  condition   known 
I  as  presbyopia,  it  is  seldom  necessary  to  use  a  cycloplegic. 
If   a  cycloplegic   is  necessary  in  presbyopia,  one  of  the 
weaker  drugs  is  generally  employed. 

^In  the  selection  of  a  cycloplegic  the  surgeon  must  be 
guided  by  the  patient's  occupation,  age,  the  character  of 
the  eyes,  and  the  refraction,'  From  the  foregoing  table  it 
will  be  seen  that  atropin  and  daturin  are  slow  in  passing 
from  the  eye,  making  their  employment  on  this  account 
very  objectionable  in  many  instances.  The  accommodation 
returns  sooner  after  the  use  of  hyoscyamin  and  duboisin 
than  from  atropin,  but  not  so  promptly  as  from  scopolamin 
and  homatropin.  The  effect  of  the  latter  is  very  brief.  A 
patient  who  might  lose  his  business  position  if  he  remained 
away  from  work  for  more  than  a  week  could  not  afford  to 
have  atropin  or  daturin  used  in  his  eyes,  whereas  a  school 
child  might  accept  atropin  as  a  luxury.  The  man  of  busi- 
ness, the  cashier  in  a  bank,  the  storekeeper,  and  others 
must,  in  many  instances,  have  their  eyes  refracted  in  at 


212        REFRACTION  AND  HOW  TO  REFRACT. 

>  least  two  days  ;  and  this  latter  time  means,  of  course,  the 
use  of  homatropin.  The  nearer  the  age  to  forty  years,  the 
jless  need  for  one  of  the  stronger  cycloplegics,  as  the  power 
of  accommodation  has  markedly  diminished  at  this  period 
of  life,  so  that  hyoscyamin  or  scopolamin  will  answer  every 
purpose.  After  thirty-five  years  homatropin  can,  as  a  rule, 
be  relied  upon  as  a  cycloplegic. 

In   hyperopic  eyes  of  young  subjects  it  is  useless  to  em- 


ploy  homatropin,  as  the  active  ciliary  muscle   requires  a 


strongly  acting  cycloplegic  to  stay  the  accommodative 
power.  In  myopic  eyes  one  of  the  stronger  cycloplegics 
may  be  used  to  advantage,  for  the  following  reasons : 
Myopic  eyes  have  large  pupils,  as  a  rule,  and  do  not  mind 
the  mydriasis  ;  myopic  eyes  are  often  in  a  state  of  irritation, 
and  the  drug  gives  them  a  much-needed  rest ;  the  myope's 
distant  vision  is  not  disturbed  by  the  cycloplegic,  as  in  the 
case  of  the  hyperope. 

Whenever  a  cycloplegic  is  prescribed,  the  patient  should 
be  ordered  a  pair  of  smoked-glass  spectacles  to  wear  dur- 
ing the  mydria^sis.  Of  the  two  forms  of  smoked  glasses, — 
r  coquilles  and*' plane, — the  latter  should  always  be  preferredt 
as  they  are  without  any  refractive  quality,  whereas  coquilles 
have  some  form  of  refraction  that  may  act  very  injuriously. 
Another  reason  for  ordering  the  plane  glass  is  that  the 
patient  will  often  wish  to  wear  them  with  his  prescription 
glasses,  which  he  could  not  do  so  well  if  they  were  coquilles. 
Dark  glasses  are  of  four  shades  of  "  London  smoked  " — A 
B,  C,  and  D,  A  being  the  lightest  shade  and  D  the  darkest. 
The  prescription  would  be  : 

For  MR.  BROWN  : 

^ >     R .     One  pair  plane  London  smoked  "  D." 

SlG. — For  temporary  use.  DR.  

March  14,  1899. 


CYCLOPLEGICS.  2  I  3 

The  cycloplegics  above  mentioned  for  purposes  of  re- 
fraction are  ordered  in  the  following  strengths  : 

Atropin.  sulphatis, gr.    j  to  aq.  dest.,  .  .  f^ij. 

Duboisin.  sulphatis, gr.  ss  "    "       "       .  .  f  3  ij. 

Hyoscyamin.  sulphatis, gr.  ss   "    "       "       .  .  f  3  iss. 

Daturin.  sulphatis, gr.  ss   "    "       "       .  .  f  5  ij. 

Scopolamin.  hydrochlor.,    .    .    . 

All  these,  except  scopolamin,  are  ordered  to  be  used 
'"7  three  times  a  day,  preferably  after  meals  ;  but  scopolamin 
being  a  very  powerful  drug,  the  surgeon  should  ^place  it  in 
the  patient's  eyes  himself  in  the  office,  and  not  give  a  pre- 
scription for  it.  Only  two  drops  are  necessary,  and  are 
instilled  a  half-hour  apart,  the  static  refraction  being  esti- 
mated one  hour  after  using  the  second  drop. 

How  to  Use  Homatropin. — This  drug  is  expensive,  and 
j  it  is  never  necessary  to  prescribe  more  than   one  grain  for 
any  one  patient.      Personally,  the  writer  has  found  the  fol- 
lowing most  satisfactory,  though  the  strength  of  the  homa- 
tropin may  be  increased  if  desired  : 

For  Miss  ROBINSON  : 

*~1  .>»  t 

li  .     Homatropin  hydrobromate, gr.  j 

Aq.  dest., n\,  xl. 

M.      Ft.  sol.      Label,  poison  drops  !  ! 
SlG. — One  drop  in  each  eye,  as  directed. 

R.     Dropper.  DR.  

March  14,  1899. 

One  drop  of  this  solution  instilled  into  a  healthy  eye  will 
produce  mydriasis  in  a  few  minutes,  but  its  action  on  the 
ciliary  muscle  is  so  trifling  that  the  near  point  will  be  but 
slightly  changed.  It  is  thus  shown  that  this  drug  is  a 
decided^fnydriatic,  and  only  becomes  a  cycloplegic  under 
definite  usage. 

To  produce  cycloplegia  with  homatropin,  the  patient  is 


214        REFRACTION  AND  HOW  TO  REFRACT. 

given    the   above   prescription  and  told  to  use  it  as  fol- 
lows :  *, 
!     To  place  one  drop  in  each  eye  at  bedtime  the  first  night. 

"This  one  drop  dilates  the  pupil  and  establishes  a  change  in 
the  circulation  of  the  blood-supply  to  the  iris  and  ciliary 
body — a  very  important  matter  for  the  patient's  comfort,  and 
at  the*  same  time  preventing  a  tendency  to  spasm  of  the 
ciliary  muscle.  The  next  morning  one  drop  is  to  be  placed 
in  the  eye  every  hour,  from  the  time  of  rising  until  leaving 
( ;  home  to  go  to  the  surgeon's  office.  At  the  office  one  drop 
is  placed  in  each  eye  about  every  five  minutes,  until  six 
drops  have  been  used ;  then,  after  waiting  half  an  hour! 

'  (for  the  cycloplegic  effect,  which  will  last  for  one  hour),  the| 
refraction  is  carefully  estimated.     After  a  short  interval  the 
cycloplegic  effect  will  begin  to  rapidly  disappear,  so  that  the 

/patient  will  be  able  to  read  within  forty-eight  hours'  time 

!with  his  correcting  glasses. 

Occasionally,  a  busy  patient  will  insist  upon  having  his 
eyes  refracted  during  his  first  visit,  and  can  not  take  time 
to  use  the  drops  in  the  manner  above  suggested.  The 
surgeon  must,  therefore,  start  and  use  the  drops  in  his 
office.  This  is  forcing  the  ciliary  muscle  into  a  state  of 
paralysis  that  does  not  always  give  ultimately  satisfactory 
results.  "  Forcing  "  homatropin  into  an  eye  in  this  way 
will  always  produce  a  "  bloot-shot "  eye  (hyperemia  of  the 
conjunctiva,  etc.)  that  does  not  improve  a  patient's  appear- 
ance ;  and  it  often  produces  severe  neuralgic  headache  that 
may  result  in  nausea  or  vomiting  in  occasional  instances. 

'Furthermore,  it  is  possible,  with  a  drug  like  homatropin,  if 
7  not  properly  used,  to  have  some  of  the  sphincter-fibers 
become  paralyzed  while  others  may  remain  free  to  act.  In 
this  way  a  spasm  of  the  ciliary  muscle  may  be  produced 
that  will  give  a  false  astigmatism.  Personally,  the  writer 


CVCLOPLEGICS.  215 

is  not  partial  to  this  method  of  forcing  the  ciliary  muscle 
into  repose. 

To  somewhat  obviate  the  "blood-shot"  condition  of  the 
eye,  and  also  to  assist  the  action  of  the  "  forcing"  process, 
one  drop  of  a  two  or  four  per  cent,  solution  of  cocain 
•  maybe  instilled  while  the  homatropin  is  being  used.  This  ' 
also  diminishes  the  danger  of  spasm.  But  cocain  is  objec- 
tionable...^ that  it  will,  in  some  cases,  "haze"  the  cornea. 
The  retinoscope  will  show  this,  and  the  patient  will  state 
that,  u;hji|e  he  can  see  the  letters  on  the  test-card,  yet  they 
have  a  "mist"  over  them.  Instead  of  using  the  hom- 
atropin alone,  a  small  amount  of  cocain  may  be  added  to  the 
solution  for  the  purpose  mentioned.  Or,  homatropin  may 
be  combined  with  cocain  and  chlorid  of  sodium  in  the  form 
of  a  disc/andone  of  these,  placed  in  the  conjunctival  sac,  is 
allowed  to  dissolve,  and  in  this  way  paralyze  the  accommo- 
dation. Or,  homatropin  may  be  used  in  a  solution  of  dis- 
"'•tilled  castor  oil.  It  is  claimed  that  when  the  drug  is  used 
in  this  form,  it  remains  in  contact  with  the  tissues  and  acts 
more  energetically. 

Homatropin  as  a  cycloplcgic  should  be  held  in  reserve 
for  individual  cases,  and  not  used  as  a  routine  practice.  It 
>is  a  good,  reliable  paralyzer  of  the  accommodation  in  many 
eyes  at  the  age  of  thirty-five,  or  thereabouts  ;  but  in  a  young 
hyperopic  eye  it  is  a  waste  of  time  to  attempt  successful 
paralysis  with  it,  and  the  danger  of  producing  a  false  "astig- 
matism should  certainly  deprecate  its  use  in  these  cases. 
Another  very  serious  objection  to  its  use  is  that  before  the 
eyes  can  become  accustomed  to  the  prescription  glasses, 
the  ciliary  muscle  recovers  and  begins  to  accommodate, 
with  the  result  that  the  patient  says  he  can  see  better  at  a 
distance  without  his  glasses  than  he  can  with  them,  and 
has  no  small  amount  of  mistrust  of  the  surgeon's  ability, 


*k 


REFRACTION  AND  HOW  TO  REFRAC 


as  he  will  have  to  wear  his  glasses  a  long  time  before  his 
ciliary  muscle  will  relax  its  accustomed  accommodative 
efforts.  This  is  not  nearly  so  likely  to  occur  if  one  of  the 
slowly  acting  cycloplegics  is  used.  X 

The  method  of  refracting  with  one  of  the  slowly  acting 
cycloplegics,  and  then  endeavoring  to  counteract  the  effect 
with  a  solution  of  eserin,  is  not  recommended.  Temporarily, 
eserin  may  overcome  the  cycloplegic  ;  but  as  its  action  is 
only  transitory,  the  paralysis  reasserts  itself  and  will  not 
disappear  until  the  specified  time. 

Refracting  one  eye  at  a  time  with  a  cycloplegic  while  the  • 
patient  pursues  his  occupation  with  the  other  eye  is  not  a 
method    to  be    considered.     This  means  a  great  amount 
of  discomfort,  headaches,  eye-strain,  and  even  diplopia  at 
times,  during  so  prolonged  a  treatment. 

If  a  hyperopic  patient  must  occasionally  use  his  eyes  for 
near  work  while  he  has  drops  in  them,  a  pair  of  +3  or 
+4  spheres  may  be  given  for  temporary  use. 

CYCLOPLEGIA. 

Cycloplegia  is  a  paralysis  or  paresis  of  the  ciliary  muscle. 
This  condition  may  be  monocular  or  binocular  ;  it  may  be 
partial  or  complete.  /Mydriasis  may  or  may  not  accompany 
the  cycloplegia,  though  the  two  conditions  usually  occur 
together  ;  and  when  they  both  exist,  the  paresis  is  spoken 
of  as  ophthalmoplegia  interna}  Th,e  ciliary  muscle  and 
sphincter  of  the  iris  are  controlled  by  branches  from  the 
third  nerve  ;  but  these  branches  are  from  independent  cen- 
ters ;  the  fibers  going  to  the  ciliary  muscle  arise  beneath  the 
floor  of  the  third  ventricle,  in  front  of  the  fibers  which  go  to 
control  the  sphincter  of  the  iris. 

Causes.  —  Temporary  paralysis  of  the  ciliary  muscle  and 
iris,  as  already  stated,  will  result  from  the  external  or  internal 


CYCLOPLEGIA.  2  I  7 

administration  of  a  cycloplegic.  It  is  interesting,  in  many 
cases,  to  find  the  cause  and  relieve  the  patient's  anxiety 
when  the  paresis  is  due  to  one  of  the  cycloplegics.  Aside 
from  the  use  of  eye-drops,  the  question  of  external ' 
medication  (liniments,  ointments,  and  plasters)  should  be 
inquired  into,  as  also  whether  rectal  or  vaginal  supposi- 
tories containing  a  cycloplegic  have  been  used. 

Other  causes  of  this  form  of  paralysis  are  tonsillitis,  quinsy, 
diphtheria,  Bright's  disease,  rheumatism,  gout,  exhausting 
diseases,  blows  upon  the  eye,  etc.  Other  and  more  serious 
causes,  as  controlling  a  guarded  prognosis,  are  intracranial 
hemorrhage,  meningitis,  syphilis,  brain  -tumor,  etc.  In 
some  instances  the  cause  can  not  be  definitely  ascertained. 

Symptoms  and  Diagnosis. — Photophobia,  dilatation  of 
the  pupil,  and  loss  of  accommodative  power  consistent  with 
the  optic  condition  of  the  eye. 

A  myopic  eye  retains  its  vision  at  the  far  point  only ;  an 
emmetropic  eye  or  a  hyperopic  eye  wearing  correcting 
glasses  has  good  distant  vision  and  absence  of  a  near 
point ;  an  uncorrected  hyperopic  eye  has  poor  distant  and 
near  vision. 

Prognosis. — This  depends  upon  the  cause. 

Treatment. — This  must  be  symptomatic  and  expectant, 
with  a  removal  of  the  exciting  cause,  if  possible.  As  many 
cases  of  cycloplegia  are  the  result  of,  or  follow,  an  attack  of 
diphtheria,  or  a  disease  which  has  reduced  the  system  below 
par,  tonics,  fresh  air,  etc.,  must  be  ordered.  When  brought 
on  by  syphilis,  mercury  and  iodid  of  potash  must  be 
prescribed.  Dark  glasses  for  the  photophobia  should 
always  be  ordered,  and  lenses  for  near-work  may  be  worn 
as  a  temporary  expedient.  The  use  of  eserin  locally  will 
occasionally  do  good  work,  but  is  not  advised  for  constant 
use  or  for  every  case.  Faradism  may  be  used  if  the  cyclo- 
19 


2l8        REFRACTION  AND  HOW  TO  REFRACT. 

plegia  is  very  persistent,  but  the  best  results  may  be  ex- 
pected from  systemic  treatment.  The  use  of  strychnin  or 
nux  vomica  are  recommended  in  certain  instances. 


OF   THE    ClLIARY    MUSCLE. 

'Cramp  of  th£  ciliary  muscle  is  the  opposite  condition  to 
that  of  cycloplegia,  just  described.  Ciliary  cramp  may 
occur  in  one  or  both  eyes,  usually  in  both  ;  it  may  occur 
in  any  form  of  ametropia  or  in  emmetropia.  Ciliary 
cramp  is  of  two  kinds  —  clonic  and  tonic. 

Clonic  cramp  is  an  occasional  and  temporary  condition 
which  comes  on  while  the  eyes  are  in  use,  and  passes  away 
soon  after  the  eyes  have  had  an  opportunity  to  rest,  and 
may  not  occur  again  for  several  days. 

-  Tonic  cramp,  also  calledf  '  spasm  "  of  the  accommodation^) 
is  a  permanent  condition  as  compared  with  the  clonic  form, 
and  occurs  whenever  the  eyes  are  used  for  distant  or  near 
vision.  The  patient  can  not  use  his  eyes  for  any  length  of 
time,  or  with  any  considerable  concentration,  without  suffer- 
ing as  a  consequence. 

Causes.  —  Clonic  cramp  may  occur  as  one  of  the  early 
symptoms  of  presbyopia.  Ametropia  is  a  very  common 
cause,  and  especially  in  cases  of  low  amounts  of  hyperopia 
or  myopia.  Emmetropia,  or  eyes  made  emmetropic  with 
glasses,  may  develop  clonic  or  even  tonic  cramp  if  the  eyes 
are  used  to  excess  or  in  a  bad  light.  Such  cases  have  been 
-called  "  hyperesthesia  of  the  retina."  Tonic  cramp  may 
develop  from  the  same  causes  which  bring  on  the  clonic 

I  form,  and  is  usually  seen  among  young  hyperopic  children, 
or  the  "pseudo-myope  "  already  described.  It  also  occurs 
occasionally  in  hysteric  patients  or  those  recovering  from 
some  severe  or  long  illness.  The  writer  has  seen  this  form 


CRAMP     OF    THE    CILIARY    MUSCLE.  2IQ 

of  cramp  precede  or  antedate  by  several  weeks  a  collapse 
of  the  nervous  system — /.  c.,  nervous  prostration. 

Symptoms. — Naturally,  ciliary  cramp  means  ocular 
pains  and  headaches.  Opera  headache,  "  train  headache," 
"  shopper's  headache,"  "  bargain-counter  headache,"  etc., 
are  some  of  the  many  names  given  to  cramp  of  the  ciliary 
muscle,  and  are,  no  doubt,  the  result  of  accommodative  effort 
in  a  bright  light  or  watching  moving  objects,  these  symp- 
toms being  a  part  of  the  history  of  accommodative  astheno- 
pia  (already  described)  and  accompanying  insufficiency  of  the 
muscles.  Symptoms  of  myopia  are  very  evident  during  the 
cramp.^  In  the  tonic  cramp  the  ocular  pains  and  headache 
may  be  so  excruciating  in  individual  cases  as  to  make  the 
family  physician  and  patient  dread  cerebral  disease  until  the 
immediate  cause  is  found  out. 

Treatment. — As  the  cause  is  usually  one  of  ametropia, 
this  must  be  corrected  by  the  careful  selection  of  glasses 
\while  the  eyes  are  undergoing  a  prolonged  rest  with  a 
t  cycloplegic  and  dark  glasses.  Later  on  the  patient  must  be 
cautioned  against  any  overuse  of  the  eyes.  The  general 
health*  should  have  any  necessary  attention.  Sedatives, 
alteratives,  and  tonics  have  their  place  in  individual  cases. 
Reflex  causes  must  be  looked  for  and,  as  far  as  possible,  re- 
moved. Insufficiencies  should  always  be  carefully  searched 
Ifor,  and  frequently  prism  exercises  to  develop  the  strength 
lof  the  weak  muscles  may  give  marvelous  results.  Un- 
fortunately, there  are  occasional  instances  of  tonic  cramp 
that  persist  in  spite  of  any  treatment,  and  such  cases  obtain 
relief  only  when  presbyopia  definitely  asserts  itself. 

Asthenopia  (from  the  Greek,  a.  priv.  ;  aOlw;,  "  strength  "; 
an}>)  "  eye  ")  means  a  weakness  or  fatigue  of  the  eye,  applying 
especially  to  the  retina,  the  ciliary  muscle,  the  extra-ocular 
muscles,  or  a  general  weakness  of  any  one  or  two  or  all 


2O 


2  2O        REFRACTION  AND  HOW  TO  REFRACT. 


of  these  structures  in  one  and  the  same  eye.     Asthenopia^ 
is  a  disease,  and  is  often  spoken  of  as  "weak  sight,"  "  eye- 
jtrain."  or  "  eye-stretching/' 

Varieties.  —  For  purposes  of  study,  differential  diagnosis, 
and  treatment,  asthenopia,  or  eye-strain,  has  been  divided 
into  the  following  varieties  :  Retinal,  muscular,  accommo- 
dative, and  asthenopia  due  to  a  combination  of  any  two  or 
all  three  varieties. 

Retinal  Asthenopia.  —  This  is  the  rarest  form  of  asthen- 
opia, and  usually  occurs  in  females.  It  is  brought  about 
by  overuse  of  the  eyes  in  too  dim  or  too  bright  a  light, 
and  may  result  from  a  too  prolonged  use  of  the  eyes  at  any 
kind  of  work  or  in  any  kind  of  light.  It  may  result  from 
exposure  to  the  sun's  rays,  to  electric  lights,  or  to  light- 
ning, or  by  reflection  from  bright  objects,  such  as  snow, 
etc.  Retinal  asthenopia  may  occur  as  a  symptom  of  hys- 
teria, or  in  a  patient  whose  nervous  system  is  peculiarly 
susceptible  to  vibrations,  sounds,  and  lights  ;  in  a  patient 
whose  nervous  system  is  an  uncertain  quantity.  Such  pa- 
tients are  very  unsatisfactory  to  treat  or  even  to  examine  ; 
they  often  imagine  that  the  reflected  light  from  the  ophthal- 
moscope or  retinoscope  is  "very  hot,"  etc. 

Symptoms.  —  The  chief  symptom  is  a  dread  of  light 
"/  (photophobia),  or   photophobia  and  lacrimation  together. 

Treatment.  —  The  first  thing  to  do  is  to  remove  the 
cause,  if  this  can  be  found  ;  otherwise  the  treatment  should 
be  very  conservative.  Ametropia  must  be  corrected  and 
the  eyes  be  given  some  regular  work  ;  in  other  words,  it  is 
r\ot  good  practice  to  restrict  all  use  of  the  eyes.  The  treat- 
ment with  "tinted  glasses,"  made  so  much  of  by  the  char- 
latan to  "  gull  "  the  innocent  public,  should  not  be  ordered, 
as  the  patient  grows  accustomed  to  them  and  they  event- 
ually become  an  absolute  necessity  on  all  occasions.  Care- 


CRAMP    OF     THE    CILIARY    MUSCLE.  221 

ful  attention  to  the  general  health  is  certainly  indicated  ; 
tonics,  out-door  sports,  etc.,  should  be  prescribed  in 
individual  cases.  The  shade  of  the  trees  is  to  be  recom- 
mended in  preference  to  the  seashore  and  bright  reflection 
from  the  sand  and  water. 

Muscular  Asthenopia. — This  is  due  to  weakness  or 
fatigue  of  one  or  more  of  the  extra-ocular  muscles,  most 
frequently  the  interni  (cxophoria).  Muscular  asthenopia 
of  the  exophoric  kind  is  the  result,  as  a  rule,  of  a  want  of 
power  to  maintain  convergence.  The  symptoms  are  in 
keeping  with  a  cramp  followed  by  a  relaxation  of  converg- 
ing power.  Ocular  pains,  eyeballs  tender  to  the  touch  (per- 
chance the  internal  recti  themselves  become  sore  to  the 
touch  or  feel  sore  on  movement  of  the  eyes),  and  in  some 
cases  the  conjunctiva  and  subconjunctival  tissues  overly- 
ing the  muscles  become  hyperemic  during  or  after  the  use 
of  the  eyes,  simulating  rheumatism  of  these  structures.  In 
other  cases  dim  vision  and  diplopia  will  be  occasional  mani- 
festations. Patients  with  muscular  asthenopia  occasionally 
find  that  they  can  continue  at  near  work  by  using  one  eye, 
but  this  does  not  occur  very  often. 

Treatment. — This  resolves  itself  into  the  correction  of 
the  ametropia,  exercise  of  the  weak  muscles,  etc.  (See 
chapter  on  Muscles.) 

Accommodative  Asthenopia. — This  is  by  far  the  most 
common  form  of  asthenopia,  and  is  due  to  fatigue  of  the 
"-Iciliary  muscle  ;  it  is,  therefore,  to  be  expected  in  hyperopic 
eyes.  It  is  caused  in  various  ways  :  from  overuse  of  the 
eyes  in  too  bright  or  too  dim  a  light,  or  from  using  the  eyes 
for  too  long  a  time  in  any  kind  of  a  light.  The  best  pair 
of  eyes,  if  overtaxed,  may  suffer  from  accommodative 
asthenopia,  even  when  wearing  the  ametropic  correction. 
Or  accommodative  asthenopia  may  result  from  a  weakness 


222        REFRACTION  AND  HOW  TO  REFRACT. 

of  the  ciliary  muscle  as  a  part  of- the  general  condition  of  the 
whole  body,  and  this  may  come  on  after  or  during  some  long 
illness,  such  as  typhoid  fever.     Accommodative  asthenopia 
x^s  often  present  in  the  early  months  of  presbyopia. 

Symptoms. — The  principal  symptom  is  headache — fron- 
tal, frontotemporal,  or  fronto-occipital ;  or  this  pain  or  dis- 
comfort may  extend  into  the  neck  or  between  the  shoulders. 
The  headache  develops  during  the  use  of  the  eyes,  and 
grows  worse  if  the  effort  is  prolonged,  and  usually  ceases 


after  the  eyes  are  rested.  See  chapter  on  Hyperopia  and 
Myopia. 

Treatment. — When  glasses  are  necessary,  they  should 
be  ordered  by  the  static  refraction.  The  general  health  of 
the  patient  should  receive  careful  attention.  An  out-of- 
door  life  will  often  be  necessary,  and  in  certain  cases  the 
time  for  using  the  eyes  at  any  near-work  will  have  to  be 
very  much  restricted. 

Accommodative  with  Muscular  Asthenopia. — This 
variety  of  asthenopia  embraces  the  two  forms  just  men- 
tioned, and  its  description  and  treatment  are  included  in  both. 

Reflexes  Due  to  Eye-strain. — Among  the  symptoms 
of  the  various  forms  of  asthenopia  described  on  the  previous 
pages,  the  writer  has  avoided  any  decided  reference  to  reflex 
symptoms,  preferring  to  speak  of  these  reflexes  in  a  general 
way  under  one  heading.  Many  patients  who  suffer  from 
headaches,  ocular  pains,  etc.,  during  the  use  of  their  eyes, 
also  very  frequently  suffer  from  constipation,  indigestion, 
heartburn,  nausea,  or  even  vomiting.  Other  patients 
may  have  nervous  attacks,  a  fear  of  some  impending 
calamity,  or  they  are  irritable  or  despondent  ;  they  may 
suffer  from  insomnia,  or,  if  they  sleep,  it  is  not  a  restful 
sleep.  Others  may  have  epileptic  attacks,  nervous  twitch- 
ings,  etc.  To  just  what  extent  eye-strain  is  responsible  for 


EXAMINATION     OF     THE     EYES.  223 

these  and  many  other  reflexes  the  writer  is  not  prepared 
to  say,  though  every  ophthalmologist  has  certainly  seen 
some  cases  of  accommodative  and  muscular  asthenopia  with 
gastric  symptoms,  or  nervous  symptoms,  or  epileptic 
attacks,  or  irritable  tempers,  or  insomnia,  or  enuresis,  etc., 
in  which  these  reflex  symptoms  entirely  disappeared  after 
the  eye-strain  was  properly  treated. 

EXAMINATION  OF  THE  EYES. 

A  systematic  method  should  be  pursued  in  the  examina- 
tion of  the  eyes,  and  the  results  recorded  in  a  book  or  on 
a  card  prepared  for  that  purpose.  The  student  should  be 
a  careful  observer,  and  also  be  able  to  question  the  patient 
intelligently  for  short  and  definite  answers.  The  following 
is  an  excellent  method  of  making  records,  but  there  is  no 
arbitrary  rule,  arid  in  this  respect  each  surgeon  may  follow 
his  own  desires  : 


Date, 

Name, 


Occupation,  ...................................... 

Age,  ...................     Sex,  ..................  -     Diagnosis,  ................... 

ACCOMMODATION.  ASTIGMATISM.  MUSCLES. 

O.  D.  V  ............  p.  p. 

O.  S.  V.,      ..........  p.  p. 

History,  ............................................... 

S.  P.  (status  prasens,  "  present  condition").     Inspection,  .................. 

Ophthalmometer,  O.  D  ..........................  O.  S  .......................... 

Ophthalnuacopic  examination,  O.  D  ..........................  O.  S  .............  -  ...... 

Manifest  refraction.      Fields.      Color  sense.     R  . 


224        REFRACTION  AND  HOW  TO  REFRACT. 

The  above  record  is  rilled  out  as  the  examination  pro- 
ceeds, but  it  is  not  always  advisable  to  follow  the  exami- 
nation in  the  order  given  ;  on  the  contrary,  it  is  better,  after 
getting  the  patient's  name  and  address,  to  ask  certain  other 
questions  which  may  appear  in  keeping  with  an  individual 
case. 

1.  Occupation. — This  is  a  very  important  question,  as 
bearing   directly  upon  the  amount  and  character  of  work 
done  by  the  eyes ;  for  example,   writing,  reading,  sewing, 
music,  engraving,  weaving,    drafting,  surveying,   painting, 
typewriting,  typesetting,  sorting  colors,  etc. 

2.  Age. — This  is  of  the  utmost  importance  in  comparing 
the  range  of  accommodation  (near  point)  with  the  emme- 
tropic  condition.     Knowing  the  patient' sage  and  near  point 

^x\vill  often  give  a  diagnosis  of  the  character  of  the  refraction. 

3.  The  name  tells  the  sex,  but   the  question   really  is 
whether  the  patient  is  married,  single,  widow,  or  widower. 

/If  a  young  married  woman,  whether  she  is  nursing  a  young 
'  I  child. 

4.  History. — Under  this  heading  the  questions  should 

bear  directly  upon  the  eyes.      "  In  what  way  do  the  eyes 

/cause  trouble?"     The   usual   answer  to   this   question  is 

\tf  headache."     To  get  a  complete  history  of  the  headache, 

and  be  able  to  differentiate  it  from  headache  due  to  other 
I  causes,  the  succeeding  questions  seem  appropriate  : 

What  part  of  the  head  aches  ?  Is  it  frontal,  occipital, 
temporal,  interocular,  vertex,  or  all  over  the  head  ? 

When  does  the  headache  come  on — during  or  after 
the  use  of  the  eyes  ?  Does  it  cease  after  resting  the  eyes  ? 
Is  the  headache  worse  when  using  the  eyes  by  artificial 
light  ?  Is  the  headache  constant  ?  Is  it  periodic  ?  Is  it 
worse  at  a  certain  hour  of  the  day?  Is  the  headache  pres- 
ent when  first  waking  in  the  morning  ?  Does  the  head  ache 


EXAMINATION     OF    THE     EVES.  225 

during  or  after  attending  a  place  of  public  amusement  or 
when  shopping  ?  If  a  female,  is  the  headache  only  monthly  ? 

The  ophthalmologist  must  not  think  because  a  patient  has 
^a  headache  that  it  is  surely  and  always  due  to  the  eyes,  and 
that  glasses  are  going  to  cure  it.  It  is  for  the  ophthalmolo- 
gist to  find  out  just  what  part  the  eyes  take  in  causing  the 
patient's  discomfort,  and  not  always  expect  to  cure  with 
glasses  headaches  that  have  no  direct  relation  to  the  eyes. 

One  of  the  most  common  causes  of  headache  which 
may  be  mistaken  for  ocular  headache  is  the  "  brow  ache  " 
due  to  malaria,  but  a  history  of  previous  malarial  attacks, 
chills  and  fever,  a  residence  in  a  malarious  district,  and  the 
fact  that  it  is  periodic  in  character,  should  certainly  give  a 
clear  differential  diagnosis. 

Other  patients  may  not  consult  the  ophthalmologist  on 
account  of  headache,  but  for  a  pain  in  or  back  of  the  eyes, 
or  back  part  of  the  head,  or  between  the  shoulders,  which 
comes  on  after  any  effort  of  vision.  Others  may  complain 
of  a  feeling  of  sand  in  the  eyes,  or  a  burning  in  the  lids,  or 
a  smarting  or  itching  in  the  lid  margins,  or  excessive  lacri- 
mation,  or  a  feeling  of  drowsiness  as  soon  as  the  eyes  are 
used  for  any  length  of  time,  or  a  feeling  as  if  the  eyelids 
would  stick  to  the  eyeballs. 

The  patient's  seeing  qualities  may  develop  the  history  of 
poor  distant  vision  and  good  near  vision,  or  vice  versa ;  this 
should  be  inquired  into  very  carefully,  and  it  may  be  well 
to  ask  about  other  members  of  the  family,  if  they  have  the 
same  condition.  Or  a  history  of  the  vision  gradually  fail- 
ing or  of  a  sudden  loss  of  sight  may  be  obtained,  and  pres- 
byopic  symptoms  should  be  referred  to,  if  the  patient  is 
over  forty  years  of  age. 

If  the  patient  wears  glasses,  it  is  well  to  inquire  whether 
the}'  were  ordered  by  an  ophthalmologist  or  if  they  are 


226       REFRACTION  AND  HOW  TO  REFRACT. 

the  patient's  own  selection.  In  the  former  instance  a  record 
should  be  made  of  the  character  and  strength  of  the  lenses, 
and  whether  the  lenses  were  ordered  with  or  without 
/'drops"  in  the  eyes;  and  if  "drops"  were  used,  if  the 
effect  lasted  for  t\vo  days  or  longer  (slowly  or  quickly 
acting  cycloplegic).  Ask  how  long  the  glasses  have  been 
worn,  and  if  the  same  symptoms  are  present  that  existed 
when  the  lenses  were  previously  ordered. 

Having  made  a  note  of  the  patient's  history,  it  is  next  in 
order  to  study  the  present  condition  (status  prcescns)  : 

1.  Breadth  of  face,  its  symmetry  or  asymmetry;  inter- 
pupillary  distance. 

2.  The  eyelids,  whether  swollen,  discolored,  or  having 
red  margins. 

3.  The  eyelashes  (cilia),  whether  regular,  irregular,  or 
absent.    If  there  are  chalazia,  styes  (hordeola),  inflammation, 
moist  or  dry  secretion  at  the  roots  of  the  cilia  (blepharitis). 

4.  Inspect  the  inner  surface  of  the  lids  and  ocular  con- 
junctiva for  inflammation  or  growths. 

5.  Inspect  the  lacrimal  apparatus  in  all  its  parts. 

6.  Inspect  the  cornea  for  its  polish,  transparency,  and 
regularity. 

7.  Depth  of  the  anterior  chamber. 

8.  Iris,  its  color  and  mobility. 

9.  Pupil,  its  size,  shape,  and  position. 

10.  Color  of  reflex  from  the  pupillary  area. 

1 1.  Palpate  to  measure  the  intraocular  tension. 

^  1 2.  Use  the  cover  test  at  1 3  inches  for  any  muscular 
anomaly. 

Following  this  record  of  the  history  and  present  condi- 

ition,  the  distant  vision  and  near  point  are  taken  for  each 
eye,  one  or  more  tests  for  astigmatism  are  made,  the  mus- 
cles are  tested  for  distance  (six  meters),  and  the  ophthal- 


EXAMINATION     OF    THE     EYES.  22"J 

mometric  measure  of  corneal  curvature  may  be  recorded. 
Finally,  and  most  important  of  all,  the  ophthalmoscopic 
examination  is  made,  and  the  cornea,  aqueous,  lens  cap- 
sule, lens,  vitreous,  nerve  (shape,  size,  color,  cupping,  and 
vessels),  conus,  macular  region,  etc.,  and  periphery  of  the 
eye-ground  are  studied. 

Lastly,  fields   and   color  sense,  dynamic  or  manifest  re- 
fraction. 


CHAPTER  IX. 
HOW  TO  REFRACT. 

General  Considerations. — Before  placing  lenses  in  front 
of  an  eye,  the  surgeon  should  be  acquainted  with  at  least 
five  important  facts  : 

1.  The  Patient's  Age. — This  tells  at  once,  from  the 
table   on  page  70  (which   the  surgeon   should  commit  to 
memory),  what  the  near  point  will  be  if  the  eyes  are  emme- 
tropic  or  standard. 

2.  The  Near  Point. — This  will  usually  indicate  hyper- 
opia if  beyond,  and  myopia  if  closer  than,  the  emmetropic 
near  point  for  the  age. 

3.  The  Distant  Vision  in  Each  Eye. — If  very  defective, 
or  if  less  than  —  and  near  point  closer  than  the  age  calls 
for,   myopia  is  indicated.     Good  distant  vision    and   near 

:  removed  indicate  hyperopia. 

4.  The   distant  vision,   if  recorded  with  question 
marks,  usually  indicates  astigmatism. 

5.  The  Results    of    Testing  with   the  Astigmatic 
Chart. — Darkest  lines  from  XII  to  VI,  or  I  to  VII,  or  XI 
to   V,   indicate    astigmatism   (myopic)   with    the    rule;    or 
darkest  lines  from  IX  to  III,  or  VIII  to  II,  or  X  to  IV, 
indicate  astigmatism  (hyperopic)  with  the  rule. 

It  is  well  to  remember  that  about  four  patients  out  of 
js/vt/f^  (five  have  hyperopia,  or  one  patient  in  five  has  myopia,  and 
the   minus   sphere    selected    almost    invariably   requires  a 
cylinder  in   combination.      Remember,  also,  that  astigma- 
;  tism  is  usually  with  the  rule  and  symmetric,  and  that  plus 

228 


HOW     TO     REFRACT.  2  29 

cylinders  are  generally  selected  at  axis  90  or  within  45 
degrees  either  side  of  90,  and  minus  cylinders  are  gen- 
erally selected  at  axis  180  or  within  45  degrees  either  side 
of  1  80. 

The  Placing  of  Trial-lenses.  —  i.  These  should  always 
be  placed  as  close  as^  ]3ogg^g^o_the_eYes  without  interfer- 
ing with  the  lashes  ;  and  to  accomplish  this,  the  trial-frame 
should  be  easy  of  adjustment. 

2.  The  center  of  the  trial-lens  must  be  opposite-  to  the 
center  of  the  pupil. 

3.  If  the  distant  vision  is  very  defective,  —     -, 


--,  or  ~l.,  —  a  strong  lens  of  2,  3,  or  4  D.  will  often  be  re- 
quired ;  whereas,  if  the  vision  is  ^—^  or  ~~,  a  weaker  lens 


would  be  called  for. 

4.  When  a  spheric  lens  placed  before  an  eye  improves  the 
vision,  it  should  not  be  changed  for  another  unless  the  vision 
is  made  better  by  having  its  strength  increased  or  dimin- 
ished by  placing  in  front  of  it  another  sphere  (plus  or 
minus)  of  less  strength.  For  instance,  if  a  -f  2  sph.  has 
been  placed  before  the  eye  and  the  vision  is  improved  from 
xY  to  vHIs  '  ^s  +  2  sPk-  snould  not  be  changed  until  a 


.50  or  —  0.50  sph.  has  been  held  in  front  of  it  and  the 
patient  states  whether  he  can  read  more  with  it  or  less  without 
it.  When  a  vision  of  -^j-  is  approximated,  then  its  accuracy 
must  be  determined  by  placing  first  a  +0.25  and  then  a 
—  0.25  in  front  of  the  correction,  so  as  to  learn  from  the 
patient  which  one,  if  either,  of  these  lenses  improves  the 
vision.  Or  if  the  correcting  lenses  selected  are  weak  ones, 
then  o.  12,  plus  and  minus,  may  be  used  in  place  of  the 
0.25. 

/     5.   Spheric  lenses   should  always   be   tried  before  using 
cylinders,  and  the  vision  brought  as  low  as  possible  with 


LAA/^-A-"  *"*£>-  '"\ 

J^U>WA^  WAJUX  IM<L  -f  4  - 

( 

23O        REFRACTION  AND  HOW  TO  REFRACT.  U\ 

a  sphere  before  combining  a  cylinder,  and,  in  fact,  after 
the  vision  has  been  improved  as  much  as  possible  with 
a  sphere,  the  pointed  line-test  for  astigmatism  may  be 
brought  into  use,  as  very  often  low  errors  of  astigmatism 
are  not  recognized  until  this  point  in  the  refraction  has 
been  reached.  ^Advocates  of  the  ophthalmometer  place 
the  cylinder  before  the  patient's  eye  and  then  add  the 
spheric  correction.  The  writer  is  not  partial  to  this  method 
or  way  of  refracting^) 

6.  When  a  patient  miscalls  one  or  more. letters  in  a  cer- 
tain  line,  the  surgeon  must   not  hurry  on  until  these  are 
corrected    by  the    patient   with    a    suitable    glass,   and  in 
this  way  the  refraction  is  gradually  worked  out  until  the 
vision  is  brought  to  the  greatest  acuity  possible.       It  is 
never  wise  to  stop  with  a  vision  of  -^j-t  as  we  are  often  able 
to  get  a  visual  acuity  of  ~,  or  occasionally  ~. 

7.  Cylinders. — When  a  plus  cylinder  is  employed,  it  is 
placed  with  its  axis  at  90,  and  then  slowly  revolved  (if  nec- 
essary) to  an  axis  where  the  patient  says  he  can  see  better. 
A  minus  cylinder  is  placed  at  axis  180,  and  revolved  in  the 
same  manner.     The  rule  (4)  for  changing  spheres  also  ap- 
plies to  cylinders — i.  e.,  to  increase  or  decrease  the  strength 
of  the  cylinder  by  placing  in  front  of  it  a  plus  or  minus 

1  cylinder  of  less  strength  at  the  same  axis. 

8.  Axis  of  the  Cylinder. — When  a  patient  is  not  sure 
about  an  exact  axis,  though  he   is  sure  that  the  cylinder 
improves  the  vision,  then  the  surgeon(may  employ  a  sphere 
of  the  strength  of  the  sphere  and  cylinder  combined,  and 
use  a  cylinder  of  the  same    strength   as  before,  but  with 
opposite  sign  and  at  about  the  opposite  axis\  For  example, 
with  +2.25  sph.  O  +0.75  cyl.,  the  patient  is  not  sure  if 
the  vision  is  best  with  the  axis  at  35,  40,  or  45,  the  sur- 
geon must  then  use  a  +3.00  sph.  O  — 0.75  cyl.,  when  the 


HOW    TO    REFRACT.  23  I 

exact  axis  (at  right  angles)  will  usually  be  selected  without 
any  hesitancy  or  doubt. 

9.  Proving  the  Correction. — All  tests  at  the  trial-case, 
when  a  cycloplegic  is  used,  should  be  confirmed  with  the 
retinoscopc. 

10.  Crossed  Cylinders. — This  is  a  trial-lens  that    has 
one  meridian  minus  and  the  opposite  meridian  plus.      They 
are  made  of  any  strength,  but  for  general  use  the  0.25  cyl- 
inders are  employed — /.  e.,  —0.25  sph.  O  +0.50  cytl The 
purpose  of  the  crossed  cylinders  is  to   increase  the  refrac- 
tion in   one  and  diminish  it  in  the  opposite  meridian.      For 
example:    if    +2.00  sph.  O+i-OO  cyl.    axis  90  gives  a 
vision  of  yggg,  and  the  crossed  cylinder   lens   is   placed  in 
front  of  this  combination  with  — 0.25  at  axis  180,  and  the 
-(-0.25    at  axis  90,  and  the  vision   comes  down  to  — ,  it 
shows  that  the  vertical  meridian  was  0.25  too  strong,  and 

j'the    horizontal    0.25    too  weak,  and  the   result  would   be 
• ,  vf'V*^' 

^ — (- 1.75  sph.  ^  +1.50  cyl.  axis  90  degrees.      Or,  if  — 3.00 


sph.  has  brought  the  vision  to  — ,  and  the  crossed  cylinder 

*!$*/'     lens   is  placed  before  it  and    rotated    to  axis    15   for    the 

*  •* 

minus  cylinder  and  axis  105  for  the  plus  cylinder,  and  the 
vision  comes  to  — ,  the  result  would  be  — 2.75  sph. 
C — 0.50  cyl.  axis  15.  C6^  v^  ^  V?  ~^vvx.|J^Lit\  fc  l^ H  r 

Methods   of    Estimating   Refraction. — To  determine     "7"M 
the  refraction  of  an  eye  it  may  or  may  not  (as  in  presby-     l>~^^ 
opia)   be    necessary  to    employ  a  cycloplegic.      When   the       f-vb 

refraction  is  estimated  without, a  cyclonlegic., it  is  spoken  of    t-t^jJLt 

•r    <  r        ,'      \     lU~  t^?"^      .- 

as  manifest  or  dynamic  (dr.,  ^jvi/^,       power    )  refraction.     /". 

When  a  cycloplegic  is  used,  the  refractive  estimate  is  spoken  ^ 
of  as  static  (Gr.  frrar-xn-,  from,  '--n^a:,  "  to  stand  at  rest  ").     In 

\  «^.^-*-»  ' 

one  instance  the  ciliary  muscle  is  permitted  to  act,  and  in         ^ 
the  other  it  is  at  rest.      A  third   method   is   to   obtain   the     / 
static  refraction  and  then  to   estimate  the  strength  of  the 


232         REFRACTION  AND  HOW  TO  REFRACT. 

glasses  to  be  prescribed  after  the  effect  of  the  cycloplegic 
has  passed  out  of  the  eyes  ;  this  is  spoken  of  as  post- 
cycloplegic  refraction.  Eyes  for  refraction  are  divided  into 
general  classes,  according  to  the  age  of  the  patient.  In 
^,-those  under  forty-five  years  of  age  a  cycloplegic  is  usually 
employed,  but  after  this  age  a  cycloplegic  is  often  dispensed 
with.  (See  Presbyopia.) 

Fogging  Method. — This  method  simulates  the  static  or 
cycloplegic  method,  as  the  ciliary  muscle  is  in  great  part 
(if  not  entirely)  placed  artificially  at  rest  by  having  in  front 
of  the  eye  under  examination  a  phis  sphere  of  sufficient 
strength  to  more  than  overcome  any  ciliary  muscle  poivcr  tJiat 
the  eye  might  otherwise  use  ^vhen  looking  at  a  distance  of  six 
meters.  The  "fogging"  method  is  so  called  because  dis- 
tant vision  is  made  obscure  or  "foggy."  The  eye  under 
examination  with  this  strong  plus  sphere  in  front  of  it  is,  to 
all  intents  and  purposes, — for  the  time  being,  at  least, — 
myopic.  This  fact  should  be  carefully  borne  in  mind,  as  the 
method  to  pursue  in  estimating  the  refractive  error  is  to 
proceed  the  same  as  in  any  regular  case  of  myopic  refrac- 
tion. Therefore,  this  method  is  only  of  service  in  estimat- 
jing  the  refraction  of  eyes  that  have  some  form  of  hyperopia 
'or  simple  myopic  astigmatism,  and  is  not  of  service  in 
myopia  or  compound  myopia. 

How  to  Proceed  in  Hyperopia. — Having  estimated 
with  the  ophthalmoscope  that  the  eye  is  hyperopic  about 
2.50  D.,  then  place  a  plus  4  D.  sphere  before  the  eye  and 
have  the  patient  look  at  the  card  of  test  letters  at  a  distance 
of  six  meters.  This  has  the  immediate  effect  of  making 
distant  vision  very  "foggy,"  but  by  waiting  a  few  seconds 
this  foggy  vision  will  clear  slightly  and  the  ciliary  muscle 
will  relax  a  part,  if  not  all  of  its  effort  to  accommodate. 
Then  proceed  as  if  refracting  a  myopic  eye  by  placing  a 


* 


^| 


> 

HOW    TO    REFRACT.  233 

—  0.50  sphere  in  front  of  the  +4  1).  sphere,  and  gradually 
increase  the  strength  of  the  minus  sphere  until  the  patient 
reads  ^.|.      If  the  minus  sphere  is  1.25  D.,  then  the  amount 
of  the  hyperopia  will  be  -f  2.75  D.,  which  is  the  difference 
between  the  +4  D.  and  the  —  1.25  D. 

How  to  Proceed  in  Simple  Hyperopic  Astigmatism. — 
Having  estimated  with  the  ophthalmoscope  or  retinoscope 
or  ophthalmometer  or  any  of  the  various  ways  that  the  eye 
has  hyperopic  astigmatism  of  about  2.50  D.,  proceed  as  in 
hyperopia  by  making  the  eye  myopic  by  the  addition  of  a 
+  4  D.  sphere.  Have  the  patient  look  first  at  the  card  of 
test  letters  and  then  at  an  astigmatic  clock-dial  next  to  the 
letters.  Proceed  by  adding  minus  spheres  as  in  the  previ- 
ous case,  and  as  soon  as  the  patient  recognizes  one  series 
of  lines  on  the  clock-dial  as  much  darker  than  those  at 
right  angles  to  these  dark  lines,  then  place  minus  cylinders 
in  position  with  their  axes  at  right  angles  to  the  darkest 
lines,  and  as  soon  as  the  lines  are  uniformly  black  on  the 
dial  then  have  the  patient  look  at  the  test  letters  again  and 
increase  the  strength  of  the  minus  sphere  if  necessary  until 
the  eye  can  read  ^j  or  ^.  Presuming  that  —  2  sphere  and 

—  2  cylinder  at  axis  I  To  degrees  were  used,  then   the  dif- 
ference between  these  and  the  -j-  4  sphere  would  give  -j-  2 
cylinder,  axis  90,  as  the  amount  of  the  hyperopic  astigma- 
tism. 

How  to  Proceed  in  Compound  Hyperopic  Astigma- 
tism.— If  the  meridian  of  highest  refraction  is  4  D.,  then 
place  a  -f  5  D.  sphere  before  the  eye,  wait  a  few  seconds, 
then  begin  by  adding  minus  spheres  until  a  series,  of  lines 
on  the  clock-dial  show  up  very  black,  then  add  minus 
cylinders  until  all  the  lines  become  uniformly  black,  then 
turn  to  the  test  letters  and  increase  the  strength  of  the  minus 
sphere  as  necessary  until  the  vision  is  brought  to  normal. 


234         REFRACTION  AND  HOW  TO  REFRACT. 

With  -f-  5  D.  sphere  before  the  eye  and  if  —  I  sphere  and 
—  3  cylinder  at  axis  165  were  employed,  then  the  com- 
pound hyperopia  would  be  -f-  1  sphere  with  -}-  3  cylinder  at 
axis  75. 

How  to  Proceed  in  Mixed  Astigmatism.  —  Estimating 
the  refraction  approximately  as  -f-2O  —  3,  cylinder;  place 
-(-  5  D.  sphere  in  position  as  before,  then  find  the  meridian 

I  of  darkest  lines  on  the  clock-dial,  add  minus  cylinder  with 
the  axis  at  right  angles  to  the  darkest  lines,  then  when  all 
lines  are  equally  black,  diminish  the  strength  of  the  plus 
sphere  until  vision  comes  to  the  normal,  if  it  is  possible  to 
get  it  to  this  point.  The  result  may  be  -f-  5  sph.  O  — 
2.50  sph.  O  —  3-  SO  cyl.  X  1  80,  which  would  equal  -{-2.50^ 
C  —  3.50  cyl.  X  1  80°. 

Advantages  of  the  Fogging  Method.  —  It  is  one  of  the 
best  approximate  ways  of  estimating  the  refraction  if  for 
any  reason  a  cycloplegic  can  not  be  employed,  as  in  cases 

of  glaucoma,  nursing  n>otners,  or  of  an  individual  who  may 

'A      i      •      TI     r       •  *u   A 

have  an  idiosyncrasy  for  a  cycloplegic.    1  he  fogging  method 

also  has  the  advantage,  like  the  cycloplegic  method,  of  un- 
covering or  bringing  out  most  if  not  all  the  latent  hyper- 
opia. 

Manifest  or  Dynamic  Method.  —  This  method  is  the 
very  reverse  of  the  "fogging"  method,  in  that  the  refraction 
is  estimated  without  first  making  the  eye  artificially  myopic, 
or  placing  the  ciliary  muscle  at  rest.  It  is,  therefore,  a 
method  thatns  liable  to  give  all  sorts  of  erroneous  results 
if  the  subject  is  hyperopic  and  less  than  forty  years  of 


o  estimate  the  refraction  by  the  manifest  method,  the 
patient  is  told  to  look  first  at  the  test  letters  and  then  at  the 
clock-dial  at  6  meters  distant  ;  the  astigmatism  is  corrected 
and  then  plus  or  minus  spheres  added  as  necessary  to  bring 


HOW    TO    REFRACT.  235 

the  vision  to  the  normal.  The  rule  is  to  employ  the 
strongest  plus  lenses  or  the  weakest  minus  lenses  which 
will  give  normal  vision. 

Advantages  of  the  Manifest  Method. — This  is  the 
method  by  which  the  eyes  of  patients  past  forty-five  years 
of  age  are  refracted  ;  a  fairly  good  method  in  cases  of 
compound  myopia  in  young  subjects  if  drops  can  not  be 
employee!. 

Objections  to  the  Manifest  Method. — It  is  not  a  method 
to  be  used  as  a  rule  in  young  subjects.  If  a  young  subject 
must  be  refracted  without  drops,  then  the  fogging  method 
should  be  followed. 

The  habit  of  prescribing  glasses  from  the  manifest  or 
"fogging"  method  without  any  knowledge  of  the  ophthal- 
moscopic  findings  is  not  a  method  that  merits  the  attention 
of  the  conscientious  physician.  Such  work  is  very  unsatis- 
factory, often  leading  to  gross  errors,  ultimate  dissatisfaction 
on  the  part  of  the  patient,  or  injury  to  the  eyes. 

Postcycloplegic  Refraction. — The  ordering  of  glasses 
after  the  static  refraction  has  been  recorded  and  the  effect  of 
the  cycloplegic  has  left  the  eyes.  For  instance  :  While  the 
ciliary  muscle  is  paralyzed  with  atropin  the  static  refraction 
is  found  to  be  -f-  I-5O  sph.  O  +2.OO  cyl.  axis  90  degrees, 
which  gives  a  vision  of  —-.  The  atropin  is  then  stopped 
>dhd  the  patient  told  to  report  in  fifteen  days,  when  the  ciliary 
muscle  will  have  regained  its  original  strength  and  gone 
back  to  its  old  habit  of  accommodating  for  distance.  The 
static  refraction  is  placed  before  the  eyes,  and  the  strength  of 
the  sphere  is  gradually  reduced  until  the  vision  just  equals 
— -,  as  it  was  when  the  "  drops  "  were  in  the  eyes.  What- 
ever this  correction  with  the  glasses  may  be,  is  ordered. 
Occasionally  the  strengtlvof  the  cylinder  as  well  as  its  axis 
is  also  changed 


236       REFRACTION  AND  HOW  TO  REFRACT. 

Objections  to  the  Postcycloplegic  Method. — The  pa- 
tient is  annoyed  by  the  long  delay  to  which  he  ie  subjected 
before  getting  his  glasses.  But  the  principal  fault  lies  in 
the  fact  that  the  eye  is  not  placed  in  the  emmetropic  condi- 
tion ;  it  is  allowed  to  retain  more  or  less  of  its  accommo- 
dative power  for  distance.  This,  however,  can  not  always 
be  avoided. 

Static  Refraction. — By  this  method  the  glasses  are  pre- 
scribed while  the  ciliary  muscle  is  under  the  effect  of  the 
cycloplegic.  In  hyperopia  allowance  must  be  made  in  the 
strength  of  the  sphere  for  the  distance  at  which  the  test  is 
made.  At  6  meters  0.25  is  deducted  from  the  sphere 
f\  without  any  change  in  the  cylinder.  The  only  possible 
^objection  to  this  method  is  in  cases  of  hyperopia,  in  which, 
after  the  effect  of  the  cycloplegic  passes  away,  the  ciliary 
muscle  may  endeavor  to  accommodate  for  distance  with 
the  glasses  in  position,  and  with  the  result  that  the  pa- 
tient can  not  see  clearly  except  near  at  hand.  To  avoid 
any  such  contingency  the  surgeon  will  have  to  make  a 
deduction  in  the  strength  of  the  plus  sphere  to  meet  such 
cases.  The  rule  for  ordering  glasses  by  the  static  refrac- 
tion in  hyperopia  is  to  deduct  0.25  from  the  sphere  and 
/  have  the  glasses  worn  at  once  and  constantly  while  the 
\effect  of  the  "  drops  "  is  gradually  leaving  the  eyes.  In 
this  way  the  eyes  grow  accustomed  (slowly)  to  seeing  at  a 
distance  without  exerting  the  ciliary  muscle ;  the  eyes  are 
thus  placed  in  an  emmetropic  condition.  If  this  effect  of 
the  cycloplegic  passes  away  before  the  patient  can  obtain 
the  glasses,  it  will  be  necessary  to  use  the  drops  for  a  day 
or  so  after  the  glasses  arc  received.  Unfortunately,  how- 
lever,  some  hyperopic  eyes,  in  young  subjects  especial  ly, 
'with  vigorous  ciliary  muscles,  will  develop  a  spasm  of  the 
accommodation  for  distant  vision  which  will  make  the 


HOW    TO    REFRACT.  237 

glasses  very  annoying  on  account  of  distant  objects  look- 
ing "dim."  Such  patients  should  be  advised  of  this  fact 
at  the  time  the  glasses  are  ordered,  and  if  dim  distant 
vision  does  develop,  that  it  will  be  transitory,  and  to  per- 
sist in  wearing  the  glasses.  There  are  two  ways  of  reliev- 
-ing  this  "  dim  "  vision  if  it  should  occur  : 

1.  To  prescribe  a  weak  solution  of  atropin  (^  of  a  grain 
to  i  fluidounce),  I   drop  in  each  eye  once  or   twice  a  day, 
the  idea  being  to  slightly  relax  the  accommodation  ;  this  is 
accomplished,  but,  unfortunately,  the  mydriatic  effect  is  a 
disturbing   element  which   the  patient  will    not   submit  to 
long  enough,  as  a  rule,  to  obtain  relief. 

2.  The    better  way   is   to    make   a   compromise   in    the 
strength  of  the  sphere.      An  eye  which  has  been  in  the  habit 
of  accommodating  3,  4,  5,  or  6  diopters  for  distance,  does 
not  often  give  up  this  habit  very  gracefully,  even  if  assisted 
by  a   slowly  acting  cycloplegic,  /so  that  when   the  static 
refraction  calls  for  more  than  3  diopters,  the  surgeon  is  fre- 
quently  compelled  to  make  a  deduction  of  more  than  0.25.  ) 
There  is  no  hard  and  fast  rule  as  to  just  hm*.'  innck  shall  be 
deducted,    and    very    few    surgeons    agree    on    this    point. 
Glasses    may   be    ordered  as   follows,  the   surgeon    being 
guided  in   great  part  by  thejpatient's  age  anoK>ecupation  ; 
also  as  to  whether  there  is   esophoria  or    exophoria.      It 
will    be  found  that  cases  of  esophoria   will  accept  almost 
a  full  correction,  whereas  cases  of  exophoria  will  require  a 
\\-ry  liberal    deduction    in  the   strength  of  the   glass,  the 
patient  being  allowed  to  use  his  relative  hypcropia  : 

Static  refraction  at  6  meters  : 

-j-l.oo  sph.  or  less  deduct  o.  12  or  0.25. 

From  -fl-oo  sph.  up  to  3.00  sph.  "     0.25  or  0.50  or  0.75. 

"     +3.00  sph.  up  to  6.00  sph.  "     0.50  or  0.75  or  i.oo  or  1.50. 

"     -f  6-°°  sph-  UP  to  8-°°  and  above     "      I.  or  1.50  or  2.00  up  to  3.00. 


238        REFRACTION  AND  HOW  TO  REFRACT. 

It  is  true  that  glasses  ordered  in  this  way  do  not  leave 
the  eyes  in  an  emmetropic  condition,  and  that,  later  on,  when 
asthenopic  symptoms  redevelop,  the  strength  of  the  glasses 
will  have  to  be  increased.  But  this  method  has  two  advan- 
tages :  first,  it  gives  the  patient  his  glasses  without  any  long 
delay,  and  the  eyes  have  an  opportunity  to  become  accus- 
tomed to  them  while  the  effect  of  the  "  drops  "  is  passing 
away ;  and,  second,  the  patient  accepts  a  much  stronger 
glass  in  this  way  than  by  the  postcycloplegic  method,  which 
is  a  decided  advantage. 

The  ordering  of  lenses  in  low  errors  for  distant  vi- 
sion depends  entirely  upon  the  condition  of  the  patient's 
eyes  and  symptoms.  It  is  not  unusual  to  find  the  most 
distressing  asthenopia,  headaches,  blepharitis,  etc.,  disap- 
pear as  if  by  magic  when  corrections  are  ordered  for  small 
defects,  especially  if  there  is  astigmatism.  In  other  instances 
slight  ametropic  errors  may  not  produce  any  unpleasant 
symptoms,  and  such  a  patient  need  not  wear  the  correction 
for  distance. 

The  Ordering  of  Glasses  in  Myopia. — There  is  no 
fixed  rule  for  prescribing  glasses  in  myopia.  Each  case  is  a 
law  unto  itself,  and  should  receive  the  most  careful  consid- 
eration from  every  point  of  view.  But  as  the  student  must 
have  some  idea  as  to  how  to  proceed,  the  writer  would  sug- 
gest the  following  subdivisions  : 

J.  Myopic  eyes  which  can  with  safety  use  one  pair  of 
glasses  for  distant  and  near  vision. 

2.  Myopic  eyes  which  require  two  pairs  of  glasses — one 
for  distant  vision,  and  another  pair  for  near-work,  reading, 
writing,  etc. 

3.  Myopic  eyes  which  should  have  the  near  correction 
only. 

CLASS  i  comprises  those  cases  in  which  there  is  an  active 


HOW    TO    REFRACT.  239 

ciliary  muscle,  and  the  ophthalmoscope  shows  but  little,  if 
any,  change  in  the  eye-ground  indicative  of  stretching  (chil- 
dren, or  in  beginning  myopia).  Glasses  carefully  selected 
by  the  static  refraction  may  be  ordered  in  such  instances 
for  constant  use,  but  with  the  distinct  understanding  that 
if  any  discomfort  arises  at  any  time  they  will  be  subject 
to  change. 

CLASS  2. — Adults  who  have  not  previously  worn  their 
myopic  glasses.  In  these  cases  the  power  of  the  ciliary 
muscle  is  weak,  deficient  in  sphincter  fibers,  and,  if  forced 
into  activity,  the  patient  would  be  very  uncomfortable,  the 
eye  would  stretch,  and  the  myopia  increase,  the  tissues  in 
these  eyes  yielding  more  readily  than  in  class  I.  The 
glasses  selected  by  the  static  refraction  may  be  prescribed  for 
distance,  but  a  second  pair,  I,  2,  or  3  diopters  weaker,  must 
be  ordered  for  the  reading,  writing,  or  working  distance, 
that  the  accommodative  effort  may  in  part  be  kept  in 
abeyance.  Class  I,  if  not  carefully  watched,  may  pass 
into  class  2  and  class  2  may  pass  into  class  3. 

CLASS  3. — These  cases  require  unusually  strong  lenses, 
and  it  is  to  these  especially  that  the  term  "  sick,"  or 
"stretched"  eye  particularly  belongs.  The  ophthalmo- 
scope may  show  vitreous  opacities,  areas  of  retinochoroid- 
itis,  macular  choroiditis,  a  broad  myopic  conus,  and  even 
posterior  staphyloma.  The  eyes  are  prominent,  occupying 
much  of  the  orbital  space.  Eyes  of  such  length  are  limited 
in  their  power  of  comfortable  rotation,  and  hence  it  is  com- 
mon for  one  eye  to  diverge,  the  patient  stating  that  lie  uses 
only  one  eye  for  near  vision.  The  diverging  eye  is  usually 
more  or  less  amblyopic,  due  to  want  of  use  or  pathologic 
changes,  or  to  both.  Such  eyes  have  lost  almost  or  quite 
all  the  power  of  accommodation.  These  eyes  must  be 
placed  in  such  a  condition  that  the  desire  to  converge  and 


24O        REFRACTION  AND  HOW  TO  REFRACT. 

accommodate  is  at  a  minimum.  The  prescribing  of  glasses 
for  these  long  eyes  must  be  limited  to  the  one  pair  for  near- 
work,  and  yet  the  patient  may,  by  bringing  the  glasses 
closer  to  the  eyes,  improve  the  distant  vision  for  the  time 
being — a  sort  of  artificial  accommodation.  To  appreciate 
what  is  meant  by  this  statement  it  is  necessary  to  reconsider 
the  optics  of  a  myopic  eye.  A  myopic  eye  of  20  D.  has  a 
far  point  of  5  cm.,  and  the  minus  lens  required  to  make 
such  an  eye  receive  parallel  rays  of  light  at  a  focus  upon 
its  retina  should  be  of  such  strength  that  the  rays  passing 
through  it  would  have  a  divergence  as  if  they  came  from 
this  far  point  (5  cm.).  Such  a  lens  would  be  a  — 20  D. 
This  means,  of  course,  that  the  — 20  D.  would  have  to  be 
placed  with  its  surface  against  the  surface  of  the  cornea, 
which  is  an  impossibility.  The  usual  distance  for  a  lens  in 
front  of  the  eye  is  I  or  I  ^  cm.,  so  that  this  distance  must 
be  subtracted  from  the  distance  of  the  far  point.  In  this 
instance  I  cm.  from  5  cm.  would  leave  4  cm.,  and  this  would 
represent  25  D.  As  just  stated,  the  glasses  for  this  class 
of  patients  are  limited  to  the  one  pair  for  near-work,  and 
therefore  it  would  be  necessary  to  reduce  the  strength  of 
these  lenses  4  diopters  and  thus  prevent,  as  far  as  possi- 
ble, any  accommodative  effort.  The  patient  using  this 
— 21  D.  for  near,  can,  if  he  wishes,  improve  his  distant 
vision  at  any  time  by  pressing  the  lenses  closer  to  his  eyes. 
The  strength  of  concave  lenses  increases  as  they  are 
brought  closer  to  the  eyes,  and  diminishes  as  they  are  re- 
moved from  the  eyes. 

Caution. — The  great  danger  in  any  refraction  at  the  trial- 
case,  but  especially  in  myopia,  is  an  overcorrection,  and 
this  is  very  likely  to  occur  if  the  surgeon  is  not  extraor- 
dinarily careful  in  having  his  lenses  placed  as  close  to  the 
eyes  as  possible  while  making  the  test. 


HOW    To    K I. TRACT.  24  I 

Prophylaxis. — The  prescribing  of  glasses  for  myopic 
eyes  is  only  a  part  of  the  general  treatment  to  which  these 
"sick"  eyes  are  entitled.  If  the  treatment  stops  at  this 
point,  then  the  glasses  may  be  an  injury  instead  of  a  bless- 
ing. Myopia  once  established  may  pass  through  the 
various  classes  already  described,  and  eventuate  in  greatly 
reduced  vision  or  total  blindness  if  certain  limitation  of 
their  use  is  not  insisted  upon. 

1.  Light. — A  good, clear,  and  steady  light  is  always  essen- 
tial ;   it  should  come  from  the  left  side,  never  from  in  front. 

2.  Time. — The  length  of  time  that  myopic  eyes  may  be 
used  should  be  restricted  as  much  as  possible,  consistent 
with  their  condition  ;  that  is  to  say,  they  should  never  be 
used  after  they  become  the  least  fatigued,  and   any  use  of 
the  eyes  should  be  counteracted  by  life  in  the  open  air. 

3.  Attitudes. — The  head  should  have  as  little  inclination 
as  possible  in  reading,  writing,  or  close  work,  as  so  faulty  a 
position  invites  a  congestion  of  the  intraocular  tissues.     At 
school  or  at  home  the  book  should  be  inclined,  and  its  dis- 
tance from  the  eyes  be  regulated  by  the  size  of  the  patient. 

4.  Print. — The   use  of  small  print   or   minute  objects 
must  be  forbidden.    English  or  Gothic  type  should  be  sub- 
stituted for  Greek,  German,   and  other  characters.      Fine 
needle-work,  embroidery,  etc.,  must  be  abolished.      If  nec- 
essary, music  notes  must  be  given  up  entirely. 

5.  Health. — The  health  of  the  patient  must  be  looked 
after,  and  all  irregularities  corrected — constipation,  etc. 

These  are  a  few  of  the  major  considerations  to  which  the 
patient's  attention  must  be  drawn,  the  surgeon  being  limited 
in  his  remarks  to  the  exigencies  of  the  individual  case. 

In  conclusion  it  may  be  well  to  know  how  myopia  is 
produced,  since  it  has  been  stated  that  the  condition  is  rarely 
seen  in  young  children.  It  is  well  known  that  astigmatism 


242        REFRACTION  AND  HOW  TO  REFRACT. 

(hyperopic)  is  a  congenital  defect,  and  with  this  in  mind  it 
is  very  easy  to  appreciate  the  succeeding  steps  which  lead 
to  the  compound  myopic  condition,  showing  at  the  same 
time  the  reason  why  simple  myopia  is  so  rare  an  anomaly. 

Take  a  child  six  years  of  age  who  has  a  compound  hyper- 
opia  of  say  +0.50  sph.  O  +0.75  cyl.  axis  90  degrees  ;  this 
child  enters  upon  its  course  of  study  without  any  correcting 
glasses,  and  is  subjected  in  its  pursuit  for  knowledge  to  a 
faulty  school  desk  and  chair,  possibly  facing  a  window. 
The  print  is  defective  in  many  ways.  The  artificial  light 
for  home  study  in  the  evening  may  be  of  poor  quality,  and 
so  placed  that  but  few  of  its  rays  fall  upon  the  child's  book. 
With  these  and  other  hindrances  the  eyes  are  strained 
(stretched).  The  tissues  are  very  yielding  in  their  growing 
state,  so  that  at  the  age  of  ten  years  the  refraction  may 
show  +0.75  cyl.  axis  90.  The  +0.50  sph.  (the  axial 
ametropia)  has  disappeared  by  an  elongation  of  the  optic 
axis.  The  vertical  meridian  is  now  emmetropic.  The  same 
conditions  exist  for  the  next  three  years,  during  which  the 
number  of  studies  is  multiplied  and  the  hours  for  study  are 
prolonged  and  the  child  reaches  the  age  of  puberty ;  the 
refraction  is  now  found  to  be  — 0.50  sph.  O  +0.75  cyl. 
axis  90  degrees,  mixed  astigmatism.  In  two  years  more  the 
refraction  is  found  to  be  — 0.25  sph.  O  — 0.75  cyl.  axis 
1 80 — /.  e.,  compound  myopic  astigmatism.  From  this  time 
forward  these  eyes  progressively  stretch  and  are  subject  to 
the  stretching  process  unless  the  progress  is  stayed  with 
glasses  and  prophylactic  treatment. 

In  the  brief  detail  of  this  one  case  the  student  will  fully 
appreciate  another  important  fact — that  the  vertical  meridian 
of  the  cornea,  as  a  rule,  maintains  throughout  the  shortest 
radius  of  curvature.  This  is  abundantly  demonstrated  by 
statistics. 


HOW    TO    REFRACT.  243 

The  following  summary  of  refractive  errors  and  direction 
of  meridians  of  shortest  radius  of  curvature  in  2500  pairs 
of  eyes, — 1300  in  private  and  1200  in  hospital  work, — pre- 
pared by  Dr.  Risley  and  the  writer,  shows  the  correctness 
of  the  above  statements  :  * 

PRIVATE.  HOSPITAL. 

Monocular  astigmatism,       70       $•<>%  94       7-8J& 

Binocular  astigmatism, 1151     88.5  828     69.0 

Total  cases, 1300  1200 

Binocular  symmetric  astigmatism,      .    .    .  694  60.2%  613  74.4% 

Binocular  asymmetric  astigmatism,     .    .    .  310  26.8  158  19.8 

Heteronymous  astigmatism, 123  10.6  40       5.2 

Homonymous  astigmatism, 24       2.1  17       1.2 

Total  binocular  astigmatism,  .    .    .    .    1151  828 

Symmetric  astigmatism : 

(a)  According  to  rule  (homologous),    .      543     78.2^  559     97.7^0 

(6)   Against  rule  (heterologous),    .    .    .      151     21.8  54       2.3 

Total  symmetric  astigmatism,     .    .    .      694  613 

Asymmetric  astigmatism  : 

(a)  According  to  rule, 223     71.8%  126     79.1$ 

(t>)   Against  rule, 87     28.2  32     20.9 

Total  asymmetric  astigmatism,  ...      310  158 

DIRECTION  OF  THE  MERIDIAN  OF   SHORTEST  RADIUS  IN  ALL  CASES  OF 
SYMMETRIC  ASTIGMATISM. 

Meridian  at  90°, 57-°-f  $ 

Meridian  inclined  15°  or  less  on  each  side, 19-74" 

Meridian  inclined  from  15°  to  30°  on  each  side, 4.0 — 

Meridian  inclined  from  30°  to  45°  on  each  side, I.O 

Meridian  at  180°, 12. 0 

Meridian  inclined  15°  or  less  on  each  side, 4.0 — 

Meridian  inclined  from  15°  to  30°  on  each  side, 2.O 

Meridian  inclined  from  30°  to  45°  on  each  side,      0.5 

*  This  report  was  read  in  the  Section  on  Ophthalmology  at  the  forty-sixth 
annual  meeting  of  the  American  Medical  Association,  at  Baltimore,  Md.,  May 
7  to  10,  1895. 


CHAPTER  X. 
APPLIED  REFRACTION. 

In  estimating  the  refraction  of  any  eye  the  surgeon  will 
do  good  work  if  he  will  make  it  a  rule  never  to  be  satisfied 
until  each  eye  has  a  vision  of  -^j-  or  more,  and  if  this  visual 
acuity  is  not  attained,  to  understand  the  reason  why : 
whether  it  is  his  fault  or  the  fault  of  the  eye  itself.  It  is 
most  essential  in  every  instance  to  have  the  good-will  of 
the  patient. 

The  following  cases  are  detailed  so  as  to  demonstrate 
each  form  of  ametropia  in  all  its  phases  : 

CASE  I. — Simple  Hyperopia. — This  is  a  common  form 
of  ametropia,  occurring  about  20  times  in  100  cases : 

January  3,  1899.     JOHN  SMITH.     Age,  twenty.     Single.     Stenographer. 

°-  D-  -VT     p-  P'=  type  °'5°  D'  at  H  cm' 

O.  S.  ^1.     p.  p.  =  type  o.5o  D.  at  14  cm.  ^f\ 

Add.—  22  degrees  ;  abd.=  6  degrees. 

History. — Frontotemporal  headaches  almost  constantly, 
but  much  worse  when  using  eyes  at  near-work.  Had 
severe  headaches  when  a  school -boy.  Never  liked  to 
study  ;  preferred  out-of-door  sports. 

5".  P. — Face  symmetric,  but  narrow.  Blepharitis  mar- 
ginalis.  Irises  blue.  Pupils  round,  3  mm.  in  diam.  Eyes 
fix  under  cover. 

Oplitlialnwinctcr. — Negative. 

Ophthalmoscope. — Both  eyes  the  same.  Media  clear. 

244 


APPLIED    REFRACTION.  245 

Disc  small   and  round,  with  physiologic   cup.     All  vessels 
near  the  disc  are   seen   clearly  with    +38.      Eye-ground 
flannel -red  and  accommodation  very  active. 
Manifest  or  dynamic  refraction  : 

O.  D.  +i.25S.  =  ^f. 
O.  S.  +i.2Ss.=  ^L. 

R .     Atropin  and  dark  glasses  for  refraction. 

January  5,  1899.  Patient  seated  at  6  meters  from  test- 
card  and  small  point  of  light.  O.  D.  V.=  j^.  =  O.  S. 
V.  ~y.  Cobalt-blue  glass  shows  (each  eye  separately) 
blue  center  and  red  halo.  (See  Fig.  125.) 

Retinoscope,  with  -(-48.,  developed  point  of  reversal  at 
I  meter  for  each  eye. 

With  trial-lenses,  O.  D.  and  O.  S.  each  select  +3  S., 
which  gives  a  vision  of  -y-,  and  they  positively  refuse  to 
see  vv'  clearly  with  an  addition  of  +0.25  S.  In  other 
words,  this  +  3  S.  is  the  strongest  lens  which  each  eye  will 
accept  and  maintain  clear  distant  vision.  Tlic  rule  for 
refraction  in  hypcropia  is  to  employ  the  strongest  lens 
which  the  eye  will  accept  without  blurring  the  distant 
vision, 

To  prove  that  the  ciliary  muscle  is  at  rest,  and  that  the 
glass  selected  is  correct,  add  a  +4  S.  to  the  distance  cor- 
rection, and  the  rays  of  light  emerging  from  the  eye  must 
focus  at  the  principal  focus  of  the  added  +4  S.,  at  10 
inches  (25  cm.);  and  if  the  patient  can  read  fine  print  at 
this  distance,  the  ciliary  muscle  is  at  rest  and  the  glass  cor- 
rect. If  a  +3  S.  had  been  added  instead  of  a  +48.,  then 
the  principal  focus  would  be  at  13  inches  ;  if  +5  S.,  then 
at  8  inches,  etc. 

The  question  is,  What  glasses  shall  be  ordered  ?     The 


246        REFRACTION  AND  HOW  TO  REFRACT. 

writer  would  give  the  following  prescription,  and  instruct  the 
patient  to  stop  the  drops  and  wear  the  glasses  constantly  : 

For  MR.  SMITH. 

K.  O.  D.  +2.75  sph. 
O.  S.  -f  2.75  sph. 

Sic. — For  constant  use. 
January  7,  1899. 

January  8th  :  Glasses  from  the  optician  neutralize ;  are 
centered  and  accurately  adjusted. 

January  2ist  :  Add.  =  18  degrees.  Abd.  =  6  degrees. 
Near  point  in  each  eye  =  10  cm.  No  headache  or  dis- 
comfort  of  any  kind. 

Considerations. — The  static  refraction  as  represented  by 
.00  sph.  means  that  rays  of  light  which  pass  through 
this   lens  and  focus  at  the  fovea  diverge  from  six  meters' 
J  distance,  which  heretofore  we  have  considered  for  purposes 
Jj&      »of  calculation  as  parallel  ;  but  when  glasses  are  ordered, 
£  ^     allowance  must  always  be  made  for  this  small  amount  of 
/divergence,  and  so  0.25  is  deducted  from  the  +3  sph.,  that 
I/       the  eye  may  have  parallel  rays  focusing  on  its  retina  when 
lit  *"  looking  beyond  a  distance  of  six  meters.     To   have  been 
^       mathematically  exact,  -f-o.  12  should  have  been  deducted  in 
i  *       JL    lace  of  +0.25. 

^e  PurPose  'm  a^  cases  of  refraction  is  to  place  the  eye 
an  emmetropic  condition,  though  this  is  not  always  advis- 
able in  every  instance.  The  hyperopic  eye  naturally  accom- 
modates for  distance,  and  the  emmetropic  eye  does  not ;  then 
the  hyperopic  eye  is  made  emmetropic  when  a  spheric  lens 
permits  parallel  rays  to  focus  upon  its  fovea  without  any 
assistance  whatever  from  the  ciliary  muscle. 

Advantages  of  Atropin  in  This  and  Similar  Cases.— 
The  glasses  are  ordered  while  the  ciliary  muscle  is  at  rest. 
The  accommodation  returns  gradually.  The  eye  becomes 


APPLIED    REFRACTION.  247 

accustomed  to  seeing  at  a  distance  without  the  assistance 
of  the  ciliary  muscle.  Atropin  produces  a  physiologic  rest, 
which  the  overacting  ciliary  muscle,  disturbed  choroid,  and 
irritated  retina  require.  None  of  these  good  results  can  be 
expected  in  a  case  of  this  kind  from  the  use  of  a  "  quick  " 
cycloplegic  like  homatropin. 

SUMMARY.  —  Age  of  patient,  twenty  years.  Amplitude  of 
accommodation  is  10  D.  for  this  age. 

Near  point  is  14  cm.,  which  shows  only  7  D. 

Facultative  hyperopia  (Hf.)  equals  difference  between  10 
and  7  D.,  which  is  3  D. 

Manifest  hyperopia  (Hm.;  equals  1.25  D. 

Total  hyperopia,  or  static  refraction  (Ht.),  equals  3  D. 

Latent  hyperopia  (HI.)  equals  the  difference  between  the 
manifest  and  total,  3  D.  and  1.25  D.,  making  1.75  D. 

The  far  point,  or  conjugate  focus,  is  negative  or  virtual, 
and  lies  back  of  the  retina,  where  the  emergent  rays  (diverg- 
ing) from  the  eye  would  meet  if  projected  backward  ;  this 
point  corresponds  to  the  principal  focus  of  the  lens  which 
corrects  the  hyperopia  —  :'.  c.,  in  this  instance,  -j~3  D.,  and 
the  negative  far  point  is  therefore  at  1  3  inches. 

The  -(-2.75  makes  the  eye  practically  emmetropic  ;  the 
near  point,  after  the  effect  of  the  atropin  passes  away,  is  10 
cm.,  which  is  the  emmetropic  near  point  for  the  patient's 
age.  The  plus  sphere  selected  represents  a  shortening  of 
the  eye  of  I  mm.,  as  measured  on  the  optic  axis. 

CASE  II.  —  Simple  Myopia.  —  \Yith  the  one  exception  of 
lemmetropia,  it  will  be  found  that  myopia,  plain  and  simple, 
r>|whhout  astigmatism,  is  one  of  the  rarest  conditions  of  the 
eye  which  the  surgeon  will  meet  in  careful  refractive  work. 
About  one  and  one-half  per  cent,  of  all  patients,  by  careful 
refraction,  are  found  to  have  simple  myopia.  Therefore 
the  condition  is  not  common. 


**-"' 

—  V 


• 


ck 


REFRACTION  AND  HOW  TO  REFRACT. 


January  3,  1899.     Miss  RARE.     Age,  twenty-five  years.     Single. 

O.  D.  V.  =    £L    p.  p.  =  type  0.50  D.  at  9  cm.;  p.  r.  at  33  cm. 

O.  S.  V.  =    — -.     p.  p.  =  type  0.50  D.  at  9  cm.;  p.  r.  at  33  cm. 

i\.  i- 

Add.  1 6  degrees.     Abd.  6  degrees.      Exophoria,  3  degrees  at  13  inches. 

History. — Does  not  suffer  much  from  headache,  but  eyes 
ache  after  any  prolonged  use  at  near-work.  Never  able  to 
see  well  at  a  distance.  Always  stood  high  in  her  class  at 
school,  though  she  had  to  have  a  front  seat  to  see  the  fig- 
ures and  writing  on  the  blackboard.  Has  excellent  vision 
for  near-work  and  does  fine  embroidery.  Has  been  accused 
of  passing  friends  on  the  street  without  speaking  to  them. 
If  she  drops  a  pin  on  the  floor,  has  to  get  on  her  knees  to 
find  it.  Parsnts  do  not  wear  glasses.  Grandfather  had 
"elegant"  sight,  had  "second  sight,"  and  never  wore 
glasses.  Patient  has  postponed  getting  glasses  because 
parents  objected. 

>S.  P. — Face  symmetric.  Interpupillary  distance,  65  mm. 
Irises  dark.  Pupils  large,  round,  5  mm.  Eyes  out  under 
cover. 

OpJithalmometer. — Negative. 

OphtJialmoscope  shows  each  eye  the  same.  Media  clear. 
Disc  large  and  round.  Shallow  physiologic  cup.  Narrow 
myopic  conus  at  temporal  side  of  disc.  Choroidai  vessels 
seen  throughout  periphery  of  eye-ground  and  extending 
almost  to  nerve  margin.  All  vessels  near  nerve-head  seen 
with  — 3.  S. 

Manifest  Refraction. — Each  eye  — 3.50  sph.  gives  vision 

<"-£• 

Cobalt-blue  glass  gives  red  center  and  dark  blue  halo. 
(See  Fig.  126.) 

R.     Atropin  and  dark  glasses  for  refraction. 


APPLIED    REFRACTION.  249 

January  5,  1899:  Patient  seated  at  6  meters  from  test- 
card.  O.  D.  and  O.  S.  vision  equals  Xl.  Retinoscope with 
— 2.  S.  develops  point  of  reversal  at  I  meter  for  each  eye. 
'It  will  be  noticed  that  the  vision  in  hyperopia  with  and 
without  drops  is  decidedly  different,  whereas  in  myopia 
there  is  little,  if  any,  change.  With  trial-lenses  each  eye 
selects  separately  — 3  sph.,  which  gives  a  vision  of  -^-.  If  a 
— 2.75  sph.  is  substituted,  the  vision  falls  to  •  5.  If  a 
— 3.25  is  employed,  the  vision  remains  ~,  but  the  letters 
look  small,  black,  and  "  far  away."  The  rule  for  refraction 
in  myopia  is  to  employ  the  weakest  lens  through  which  the 
eye  can  still  maintain  clear,  distant  vision. 

What  Glass  to  Order. — The  writer  would  give  the  fol- 
lowing prescription  and  instruct  the  patient  to  stop  the 
drops  and  wear  the  glasses  constantly  : 

January  7,  1899.     For  Miss  RARE. 
U.     O.  I).  —3.00  sph. 
O.  S.    —3.00  sph. 
SIG. — For  constant  use. 

January  9th  :  Glasses  neutralize  ;  are  centered  and  accu- 
rately adjusted.  Add.  18  degrees.  Abd.  10  degrees. 
Patient  is  delighted  with  glasses. 

January  2 1st:    Near  point,  12  cm. 

Considerations. — As  a  rule,  concave  lenses  are  ordered 
without  any  deductions  for  the  slight  amount  of  diver- 
gence of  the  rays  of  light  for  the  distance  (6  meters)  at 
which  the  estimate  is  made.  To  be  exact,  — 0.25  should 
:be  added  to  the  — 3.00  sph.  in  this  case  ;  but  the  surgeon 
must  avoid  the  danger  of  tfrrrcorrecting  the  myopic  eye, 
and,  to  be  on  the  safe  side,  1(ne  glass  is  usually  ordered  as  the 
patient  selects  and  the  retinoscope  confirms  j^  These  lenses 
make  the  eyes,  to  all  intents  and  purposes,  emmetropic. 


f\ 


25O  REFRACTION     AND     HOW     TO     REFRACT. 

Advantages  of  Atropin. — The  choroid  and   retina  are 
given  a  physiologic  rest  that  they  could  not  obtain   in  any 
other  way.  (/The  patient  will  not^sgjfifctiUtflr strong  a  glass.     . 
as   was  the  case  in    this  very  instance  w Hwfc"" m aiMgyfcefrft  ^  >• 
Myopic  eyes  usually  have  large  pupils,  and  do  Jiot  suffer, 
therefore,  from  mydriasis  to  the  same  extent  as^Tr^K^s 
eyes.    The  far  point  remains  unchanged.    The  power  of  con- 
vergence is  somewhat  relieved   by  the  glasses,  \\vhich   at 
the    near  working  distance  are  of  the  nature  of  prisms, 
bases  in. 

SUMMARY. — Age  of  patient  is  twenty-fiye  years.  Ampli- 
tude of  accommodation  at  this  age  is  8  ^D.  /'Near  point,  9 
cm.  =  11  D.,  and  far  point  33  cm.  =30.  Difference  be- 
tween near  point  and  far  point  in  diopters  =  8  D.,  which  is 
the  amplitude  of  accommodation  for  the  patient's  age^/ 
I  Difference  between  the  near  point  in  diopters  (i  i  D.) 
(and  the  amplitude  (8  D.)  is  3  D.,  which  is  the  amount  of 
|the  distant  correction  needed.  With  glasses  on,  the  near 
point,  after  the  effect^ of  the  atropin  passes  away,  is  12  cm., 
which  represents  the  emmetropic  near  point  for  the  age. 
This  myopia  of  3  D.  represents  an  eye  i  mm.  longer  than 
the  standard  eye,  as  measured  on  the  optic  axis. 

CASE  III. — Simple  Hyperopic  Astigmatism. — Not  an 
uncommon  condition.  About  5.5  percent,  of  all  eyes  have 
this  form  of  refraction. 

April  3,  1899.     Miss  ROBINSON.     Age,  twenty-four  years.     Single.     Dress- 
maker. 

VI 
O.  D.  V.  =-jjj-???.     p.  p.  =  type  0.50  at  18  cm. 

O.  S.   V.=-j5Jr??  ?.     p.  p.  =  type  0.50  at  18  cm. 
Add.,  23  degrees.     Abd.,  5  degrees.     At  6  meters,  esophoria  4  degrees  ; 
at  13  inches,   I  degree  of  esophoria. 

Astigmatic  clock-dial  shows  darkest  lines  from  X  to 
IV  with  O.  D. 


APPLIED    REFRACTION.  251 

Astigmatic  clock-dial  shows  darkest  lines  from  VIII  to 
II  with  O.  S. 

History. — Headache  every  day  ;  seldom  entirely  free  from 
ocular  discomfort.  Distress  begins  in  the  forehead  and 
extends  to  the  back  of  the  head  and  into  the  neck.  After 
a  hard  day's  sewing,  has  to  go  to  bed  and  bind  the  head 
with  a  handkerchief.  Once  a  week  has  a  "  sick  headache," 
when  she  has  to  give  up  work  entirely  and  take  headache 
powders.  Sick  headache  often  ceases  after  emesis. 

S.  P. — Face  symmetric.  Blepharitis  well  marked,  with 
many  cilia  missing.  Edges  of  lids  thickened.  Irises  light 
blue  in  color.  Pupils  apparently  oval  in  vertical  meridian. 
Corneal  reflex  shows  axis  inclined  from  vertical  in  each  eye. 

OpJitlialmomctcr. — O.  D.,  3.50  ;  axis,  75,  with  the  rule  ; 
O.  S.,  3.50;  axis  105  degrees,  with  the  rule. 

Oplithalnwscopc. — O.  D.,  vertically  oval  nerve  axis,  75 
degrees.  Accommodation  very  active.  Underlying  con  us 
down  and  out.  Vessels  at  75  best  seen  with  -f-  2.50  ;  and  at 
axis  165,  without  any  lens.  O.  S.,  same  general  conditions 
as  in  O.  D.  Vertically  oval  nerve  axis,  105.  Vessels  at  105 
degrees  seen  with  -f-  2. 50,  and  at  axis  I  5  without  any  lens. 

Manifest  Refraction. — 

VI 

O.  D.,  +2.50  cyl.  axis  65  degrees  =  -yj-  ?  ?  ?. 

O.  S.,  same  cylinder  with  axis  125  degrees. 

R.      Hyoscyamin  and  dark  glasses  for  refraction. 

A/ 

April  ;th  :    Six  meters  from  test-card  and  point  of  light,  *^ 

/  t  **> 

O.   D.  «  ''     ^  <* 

•'< 


Clock-dial    shou^-flie    same    as    at    first   examination.      A <f 

^^^"^^ 

Cobalt-blue"gTass  before  O.  D.  gives  blue  center  and  red 
on  each  side  at  axis  165  degrees.  O.  S.  the  same  at  axis 
15  degrees.  (See  Fig. 

. 

.7"  /  /  -4-£>  *^i    i^t    &>4    n/£    &>*-* 
*nr***>* 


•' 


252        REFRACTION.  AND  HOW  TO  REFRACT. 

Stenopeic  Slit.  —  O.  D.,  axis  75  with  +0.25  S.,  V.  =  ~ 
and  at  axis  165  with  +2.508.,  V.  =  ~.  O.  S.,  axis  105 
with  +0.25  S.,  V.  =  —  ;  and  at  axis  15  with  +2.50  S., 

V   -  --^ 
"  vi  ' 


Rctinoscope  at  I  meter  shows  :  O.  D.,  at  axis  75  degrees 
-(-1.25  S.,  and  axis  165  degrees,  +4.25  S.  O.  S.,  at  axis 
105  degrees,  +1.25  S.,  and  at  axis  15  degrees,  -f~4-25  S. 

At  Trial-case.  — 

O.  D.  +0.25  sph.  O  -f  3.00  cyl.  axis  75  degrees,  V.  =  _Y_I  -f- 
O.  S.  +0.25  sph.  O  +3.00  cyl.  axis  105  degrees,  V.  =  .  vt.  -\-. 

April  6th  :  Same  result  as  April  5th.  Add.,  20  degrees. 
Abd.,  6  degrees.  Esophoria,  2  degrees  at  6  meters. 

For  Miss  ROBINSON. 

R.     O.  D.  -(-3.00  cyl.  axis  75  degrees. 
O.  S.  -(-3.00  cyl.  axis  105  degrees. 

Sic.  —  For  constant  use. 

> 

April  7th  :  Glasses  neutralize  ;  are  centered  and  accu- 
rately adjusted. 

April  1  6th  :  Perfectly  comfortable.  Free  from  headache 
since  the  first  day  she  used  the  "  drops."  Add.,  20  de- 
grees. Abd.,  6  degrees.  Esophoria  at  6  meters,  2  de- 
grees ;  and  at  13  inches,  o°.  Near  point,  12  cm. 

Considerations.  —  Apparently,  the  static  refraction  in  this 

case  would  indicate  compound  hyperopic  astigmatism  ;  but 

~*\   when  0.25  is   deducted  to  produce  parallel  rays,  then  the 

prescription  becomes  one  for  simple  hyperopic  astigmatism. 

General  Rule  for  Prescribing  Cylinders.  —  Order  the 
cylinder  just  as  found,  without  any  change  in  its  axis  or 
strength. 

The  vision  in  each  eye  at  the  different  visits,  before 
lenses  were  placed  in  front  of  the  eyes,  was  always  uncertain, 


APPLIED    REFRACTION.  253 

the  patient  miscalling  certain  letters,  and  hence  it  is  that 
the  vision  is  recorded  with  as  many  question  marks  as 
there  are  mistakes  in  the  line  of  letters — /'.  e.,  —  ??  ?. 

IX 

In  taking  the  vision  at  the  first  visit,  the  patient  could 
read  part  of  ^^  if  not  closely  watched.  In  other  words, 
if  she  was  permitted  to  tilt  her  head  to  one  side  and  nar- 
row the  palpebral  fissure  by  squinting  the  lids  together,  and 
making,  as  it  were,  a  stenopeic  slit  out  of  her  eyelids,  the 
vision  was  improved.  But  when  told  to  open  the  eye  wide, 
she  could  read  only  part  of  — .  This  is  explained  by 
the  fact  that  when  the  lids  were  drawn  together,  the  verti- 
cal meridian  was  partly  excluded,  and  then,  by  accommo- 
dating, the  vision  was  improved  through  the  horizontal 
meridian.  Astigmatic  eyes  often  take  advantage  of  this 
condition  when  the  nature  of  the  astigmatism  is  suitable, 
but  only  at  the  expense  of  frowning  and  straining  the 
accommodation. 

It  will  also  be  noticed  that  the  stenopeic  slit  was  not  used 
as  a  test  at  the  first  visit.  This  is  also  explained  for  the 
same  reason  that  the  patient  would  draw  his  lids  together 
and  therefore  annul  the  virtue  of  this  test.  The  stenopeic 
slit  is  to  be  used  in  these  cases  only  when  the  ciliary  muscle 
is  at  rest. 

SUMMARY. — When  the  hyoscyamin  has  passed  out  of 
the  eyes  and  the  glasses  are  in  position,  the  near  point  be- 
comes 12  cm.,  which  is  quite  consistent  with  the  patient's 
age.  <  Before  using  drops,  the  near  point  with  the  eyes  wide 
open  was  only  18  cm.,  representing  about  5.50  D.  ;  and 
this,  subtracted  from  the  amplitude  for  twenty -four  years  of 
age,  would  leave  3  D.  for  distance  uncorrected./ 

As  every  6  D.  cylinder  represents  I  mm.  of  lengthening 
or  shortening  of  the  radius  of  curvature  of  the  cornea, 
then  this  patient,  taking  a  -f-  3  cyl.  at  axis  7  5  in  the  right 


254        REFRACTION  AND  HOW  TO  REFRACT. 

eye,  has  the  165  degree  meridian  l/2  of  a  mm.  too  long  as 
compared  with  the  75  meridian,  which  is  supposed  to  have 
the  normal  radius  of  7.8  mm. 

The  same  is  true  of  the  meridians  of  the  left  eye. 

CASE  IV. — Simple  Myopic  Astigmatism. — Not  a  com- 
mon condition.  About  1.5  per  cent,  of  all  eves  have  this 
form  of  refraction.  ^^4^- 

April  10.     Miss  JENKS.     Age,  eighteen  years.     Single. 

O.  D.  V.  =  ~  ?  ?  ?-     P-  P-  9  on- 

VI 
O.   S.  V.  =  ^y-j  ?  ?  ?.     p.  p.  9  cm. 

Add.,  20  degrees.    Abd.,  5  degrees.     Esophoria  at  6  meters  =  3  degrees  ; 
and  I  degree  at  13  inches. 

Pointed  Line  Test. — Each  eye  selects  the  series  of  points 
from  XII  to  VI  as  coalescing  and  appearing  as  dark 
lines. 

Cobalt-blue  Glass. — O.  D.  and  O.  S.  each  show  blue 
above  and  below  the  red.  (See  Figs.  1 29  and  1 30.) 

Stenopeic  Slit. — Axis  90  degrees  V.  =  ~~  ;  axis    1 80  V. 

VI 
VI* 

History. — Never  had  good  distant  vision.  Has  occa- 
sional headaches.  Comes  to  find  out  if  glasses  will  im- 
prove vision. 

S.  P. — Face  symmetric.  Irises  dark  in  color.  Pupils 
apparently  round,  4  mm.  in  diameter.  Eyes  out  under 
cover. 

Ophthalmometer. — Each  eye  2  D.,  axis  90. 

Ophthalmoscope. — O.  D.,  media  clear.  Disc  large  and 
round,  with  underlying  conus  out.  No  physiologic  cupping. 
Choroidal  circulation  everywhere  recognized,  characteristic 
a  stretching  eyeball.  Horizontal  vessels  seen  with 
— 2  S.  ;  vertical  vessels  seen  without  any  correcting  lens. 
O.  S.,  same  general  conditions  as  in  O.  D. 


APPLIED    REFRACTION. 

Manifest  Refraction. — 

O.  D.,  -2.50  cyl.  axis  180  =  ^| . 
O.   S.,  —2.50  cyl.  axis  180  =  ^  [ . 
R .     Atropin  and  dark  glasses  for  refraction. 

April  1 2th  :  Six  meters  from  test-card  and  point  of  light. 
Retinoscopy  at  one  meter.  Vertical  meridian  +1.25  S. 
Horizontal  meridian  — i.oo  S. 

Stenopdc  slit  at  axis  1 80  with  -f-  o.  2  5  ==  x  '  ;  at  axis  90  = 

—so,  >:;. 

Cobalt-blue  glass  and  pointed  line  test  show  same  results 
as  at  first  visit : 

VI 
O.  D.  V.  =  ~y  ?  ?  ?. 

O.  S.  V.  =  -^V  ?  ?  ?- 
At  Trial -case. — 

O.  D.,  -j-o.25sph.  O — 2.00 cyl.  axis  180 degrees  =  V1  . 

VI 
O.  S.,  -f  0.25  sph.  ^  — 2.00  cyl.  axis  iSodegrees  =  -=77-. 

April  1 3th:  Same  results  as  yesterday.  Add.  =  20  ; 
Abd.  =  6.  Esophoria,  2  degrees. 

For  Miss  JENKS. 

R.     O.  D.,  — 2.00  cyl.  axis  180 
O.   S.,  — 2.00  cyl.  axis  180. 

April  1 4th:  Glasses  neutralize.  Centered  and  properly 
adjusted. 

April  28th  :  Comfortable.  Enjoys  good  distant  vision. 
Near  point,  each  eye,  9  cm. 

Considerations. — Apparently,  the  static  refraction  would 
indicate  mixed  astigmatism,  but  when  -(-0.25  is  deducted 
to  produce  parallel  rays,  the  prescription  resolves  itself  into 
one  for  simple  myopic  astigmatism. 

The  general  rule  for  ordering  cylinders  is  the  same  in 


256        REFRACTION  AND  HOW  TO  REFRACT. 

myopia  as  in  hyperopia — /.  e.,  no  change  in  the  strength  or 
in  the  axis  of  the  cylinder. 

A  cycloplegic  is  always  necessary  in  such  cases,  as  is 
shown  by  the  different  results  obtained  by  the  manifest  and 
stenopeic  slit. 

The  vision  was  always  uncertain  before  lenses  were  placed 
before  the  eyes,  as  is  indicated  by  the  question  marks. 

At  the  first  visit  the  vision  was  taken  with  the  eyes  -wide 
open.  If  allowed  to  narrow  the  palpebral  fissure  by  squint- 
ing the  eyelids  together  and  making  a  stenopeic  slit  out  of 
them,  the  patient  could  read  a  part  of  y^g.  When  the 
lids  were  thus  drawn  together,  the  myopic  vertical  meridian 
was  excluded  in  part  and  the  horizontal  meridian  was  util- 
ized. cThe  stenopeic  slit  was  of  some  assistance  before 
drops  were  used,  as  the  accommodation  could  not  be 
exerted,  as  in  the  case  of  the  hyperopeJ 

SUMMARY. — After  recovery  from  the  cycloplegic,  small 
type  was  clear  at  9  cm.,  which  was  in  keeping  with  the 
patient's  age.  The  near  point  before  "  drops  "  were  used 
was  also  9  cm.,  but  not  constant,  nor  was  the  type  clear. 
The  — 2. oo  cyl.  at  axis  180  represents  ^  of  a  mm.  of 
shortening  in  the  vertical  radius  of  curvature  as  compared 
with  the  normal  radius  of  7.8  mm.  in  the  horizontal. 

The  astigmatism  is  regular,  symmetric,  with  the  rule. 

CASE  V. — Compound  Hyperopic  Astigmatism. — The 
most  common  form  of  all  refraction.  It  is  a  combination 
of  simple  hyperopia  with  simple  hyperopic  astigmatism. 
About  44  per  cent,  of  all  eyes  have  this  form  of  refraction. 

April  1 2th.     MR.  COMMON.     Age,  twenty-eight.     Married.     Bookkeeper. 

VI 
O.  D.  V.  =-^-??.     p.  p.  =  type  0.75  D.  =  22  cm. 

VI 
O.  S.  V.  =  "x~??-     P-  P-  =  type  0.75  D.  =  22  cm. 

Add.,   23  degrees.      Abd,,  7  degrees.      Esophoria,  2   degrees;   at  13 
inches,  o. 


APPLIED    REFRACTION.  257 

Astigmatic  Clock-dial. — O.  D.  and  O.  S.  each  selects 
darkest  series  of  lines  from  IX  to  III. 

Phicido's  disc  shows  each  corneal  image  as  a  horizontal 
oval.  Schemer's  test  shows  two  lights,  separated  in  all 
meridians  :  the  vertical  have  the  least  separation  and  the 
horizontal  the  most. 

Ophthalmometer. — 2.25  D.  with  axis  90  in  each  eye. 

History. — Family  physician  has  tried  in  vain  to  stop  the 
headaches,  which  he  said  were  from  biliousness.  Headache 
develops  as  soon  as  the  patient  commences  to  use  his  eyes, 
and  gets  worse  toward  noon  ;  and  from  that  time  on,  during 
the  rest  of  the  day,  he  is  cross  and  irritable,  and  feels  dizzy. 
Unable  to  read  in  the  evenings  as  he  did  a  few  years 
ago.  Is  wearing  a  pair  of  "  rest  "  glasses,  which  he  received 
from  an  optician  ;  they  were  of  some  benefit  for  a  very  short 
time. 

5.  P. — Lid  margins  red  and  excoriated.  Many  fine  scales 
(looking  like  dandruff)  adhering  to  the  cilia.  Irises  gray 
in  color.  Pupils  round,  3  mm.  Eyes  in,  under  cover. 

Ophthalmoscope.  —  O.  D.  and  O.  S.  each  medium  clear. 
Disc  small,  vertically  oval.  Shallow  physiologic  cup. 
Venous  pulsation  on  disc.  Narrow  conus  to  temporal  side. 
Nerve-head  prominent  and  edges  somewhat  hazy.  No  path- 
ologic conditions  recognized.  Vertical  vessels  best  seen 
with  -f- 3.00  and  horizontal  with  -f-i.oo. 
U .  Duboisin  and  dark  glasses  for  refraction. 

April  1 4th  :  Six  meters  from  test-card  and  point  of  light. 

VI 

O.  D.  V.  =  j*x  ?  ?  ?- 

O.  S.  V.  =  ^???. 

Retinoscopy  develops  point  of  reversal  at  one  meter  in 
each  eye;  vertical  meridian  with  +2.25  S.,  and  horizontal 
meridian  with  +4.00  S. 


2$S         REFRACTION  AND  HOW  TO  REFRACT. 

Stenopeic  slit  axis  90  degrees  with  -}-  1.25  =  -^j-  ;  at  axis 
1 80  degrees  with  -f  3-OO  —  -v,-j-. 

Cobalt-blue  glass,  blue  center  and  red  all  round,  more 
conspicuous  on  the  right  and  left  sides.  (See  Figs.  131 
and  132.) 

At  Trial-case. — 

O.  D.,  +1.25  sph.  O  -fi-75  cyl.  axis  90  =  -TT?-. 
O.  S.,  -)-l.25  sph.  O -(-1.75  cyl.  axis  90  =  -¥J-. 

April  1 5th  :  Same  results  as  April  I4th.  Add.,  22.  Abd., 
7.  Esophoria,  I  degree. 

For  MR.  COMMON  : 

R.     O.  D.,  -f  i.oo  sph.  O  4  I-75  cyl.  axis  90  degrees. 

O.  S.,  4-1.00  sph.  O  +1.75  cyl.  axis  90       " 
SlG. — For  constant  use. 

April  i  /th  :  Glasses  neutralize  ;  are  centered  and  ad- 
justed. 

April  24th  :  Has  been  perfectly  free  from  headaches 
ever  since  getting  glasses.  Never  realized  what  a  blessing 
glasses  could  be.  Near  point  with  each  eye  is  now  14  cm. 

Considerations. — The  prescription  for  glasses  was  the 
same  as  the  static  refraction,  with  the  exception  of  the  re- 
duction in  the  strength  of  the  sphere.  No  change  in  the 
cylinder.  A  cycloplegic  as  a  means  of  obtaining  a  prompt, 
correct,  and  satisfactory  result  in  such  cases  can  not  be  dis- 
pensed with. 

The  decided  change  in  the  vision  before  and  with  the 
cycloplegic  is  quite  diagnostic  of  compound  hyperopic  as- 
jtigmatism.  When  the  cylinder  and  sphere  are  of  any  con- 
siderable strength,  the  patient  can  often  overcome  (faculta- 
tive hyperopia)  the  spheric  but  not  the  cylindric  correction. 

SUMMARY. — After    recovery   from    the    cycloplegic   the 


APPLIED    REFRACTION.  259 

small  type  becomes  clear  at  about  13  cm.,  which  is  the 
near  point  consistent  with  the  patient's  age.  The  far  point 
before  using  drops  was  really  two  points,  both  negative — 
that  of  the  vertical  meridian  being  about  I  meter,  and  the 
horizontal  meridian  about  ^  of  a  meter  back  of  the  retina. 

The  form  of  the  astigmatism  is  regular,  symmetric,  and 
with  the  rule. 

CASE  VI. — Compound  Myopic  Astigmatism. — A  com- 
bination of  simple  myopia  with  simple  myopic  astigmatism. 
This  is  the  usual  form  of  refraction  in  myopic  eyes.  About  8 
per  cent,  of  all  eyes  have  compound  myopia.  The  writer's 
experience  is  such  that  he  never  refracts  a  case  of  myopia 
without  searching  carefully  for  a  cylinder  in  combination 
witli  the  sphere. 

April  1 2th.     MRS.  USUAL.     Age,  thirty  years.     Married.     Housewife. 

VI 
O.    D.  V.,  --jj-  ?  ?  ?,  type  0.50  =  7  to  14  cm. 

VI 
O.    S.   V.,  -£-  ?  ?  ?,  type  0.50  =  7  to  14  cm. 

Add.,  1 6  degrees.     Abd.,  6  degrees.     Exophoria  at  6  meters,  2  degrees. 

History. — Suffers  from  ocular  pains,  as  if  knife-points 
were  sticking  into  the  eyes,  which  come  on  as  soon  as  near- 
work  is  attempted  or  continued.  Says  that  she  constantly 
sees  fine  dust  particles  floating  before  her  vision.  Has  been 
wearing  glasses  from  an  optician  ( — 3  sph.).  Has  all  the 
symptoms  of  near-sightedness.  Family  history  of  father 
and  two  sisters  wearing  glasses  for  "near  sight." 

S.  P. — Face  symmetric.  Eyeballs  prominent.  Irises 
dark  in  color.  Pupils  small  (for  a  myope)  and  round, 
3  mm.  Eyes  markedly  out  under  cover. 

Ophthalmoscope. — O.  D.,  many  fine  floating  vitreous 
opacities.  Nerve  large  and  round,  with  broad  underlying 
conus  down  and  out.  Choroidal  vessels  seen  throughout 
eye -ground.  Vessels  at  about  axis  1 20  degrees  best  seen 


26O        REFRACTION  AND  HOW  TO  REFRACT. 

with  — 2,  and  vessels  at  axis  30  degrees  best  seen  with  — 3. 
O.  S.,  same  general  conditions  as  in  O.  D.,  except  the  prin- 
cipal meridians  are  about  60  degrees  and  1 50  degrees. 

Indirect  method  shows  a  vertically  oval  nerve,  with  the 
conus  to  the  nasal  side  of  the  aerial  image  (as  the  eye- 
ground  and  nerve-head  have  undergone  vertical  and  lateral 
inversion).  Withdrawing  the  lens,  the  nerve  grows  larger 
in  all  meridians,  but  more  so  in  the  vertical. 

Cobalt-blue  glass  shows  O.  D.  red  center,  blue  all  around, 
more  pronounced  on  the  sides  in  the  120  meridian.  O.  S. 
shows  red  center,  blue  all  around,  more  pronounced  on  the 
sides  in  meridian  of  60  degrees.  (See  Figs.  133  and  134.) 

Stenopeic  slit  before  O.  D.  at  axes  1 20  V.  ^^^jFjnT  ?  ?  ?  >  at 
axis  30  V.  ="xx~  ?  ?•  O.  S.,  the  same  with  axes  60  degrees 
and  150  degrees. 

Astigmatic  Chart. — O.  D.  selects  the  lines  from  V  to  XI 
as  darkest.  O.  S.  selects  the  lines  from  VII  to  I  as 
darkest. 

Ophthalmometer. — O.  D.,  I  D.,  axis  35  degrees.  O.  S., 
I  D.,  axis  145  degrees. 

Manifest. — 

VI 
O.  D.,  — 2.5osph.  C  —0.75,  cyl.  axis  35  =  -vn  ?  ?  ?  ?. 

O.  S.,  — 2.50  sph.  O  — 0.75  cyl.  axis  145  =  -VI[  ?  ?  ?  ?. 
1£ .     Atropin  and  dark  glasses  for  rest  and  refraction. 

April  1 3th:  At  six  meters  from  test -card  and  point  of 
light : 

O.D.V.=1§-?.     0.S.V.=]g-?. 

O.  D.,  — 2.00,  sph.  O  — i. oo  cyl.  axis  30  ss  -yj-  ?  ?  ?. 

VI 
O.  S.,  — 2.00  sph.  O  — I. oo  cyl.  axis  150  =  -vj- ?  ?  ?. 

Retinoscope  confirms  this  trial-case  result.  Retinoscope 
also  shows  a  general  cloudiness  of  the  media  (vitreous), 


APPLIKI)    REFRACTION.  26 1 

which,  of  course,  will  account   in  part   for   the  vision  not 


VI 


being  -v-  in  each  eye  with  correcting  glasses. 
April  1 4th  :  Same  result  as  on  the  ijth. 

For  MRS.  USUAL. 

R.     O.  D.,  — 2.00  sph.  O  — i.oo  cyl.  axis    30  degrees. 

O.  S.,  — 2.00  sph.  O  — i.oo  cyl.  axis  150       " 
SlG. — For  distance,  as  directed. 

April  i  5th: 

K .     Tonics.     Rest  of  eyes.     Attention  to  general  health. 

April  i  /th  :  Glasses  neutralize ;  are  centered  and  ad- 
justed. 

April  2pth  :  Add.,  16.     Abd.,  6.     Exophoria,  2. 

Vision  in  each  eye  read  slowly. 

Considerations. — The  static  refraction  was  ordered  just 
as  found,  and  no  deduction  whatever  was  made  in  the 
sphere.  The  rule  is  to  prescribe  in  the  same  way  as  in 
/simple  myopia.  But  all  cases  of  myopia  can  not  and  must 
not  be  prescribed  for  by  rule.  Each  case  of  myopia  is  a 
law  unto  itself.  See  description  under  General  Considera- 
tions, page  238,  also  pages  227,  228,  and  229. 

SUMMARY. — The  near  point  is  now  14  cm.,  which  is 
perfectly  consistent  with  the  patient's  age.  Fourteen  centi- 
meters represents  an  accommodative  power  of  7  D.,  and 
this  was  the  difference  between  the  near  and  far  points 
before  the  drops  were  used.  The  astigmatism  is  regular, 
symmetric,  and  with  the  rule.  Vision  is  not  brought  up  to 
normal  on  account  of  the  changes  in  the  vitreous  and  dis- 
turbed eye -ground,  due,  no  doubt,  to  the  want  of  a  proper 
correction — the  cylinder.  The  choroid  and  retina  are  both 
in  a  stretching  condition. 

CASK  VII. — Mixed  Astigmatism. — Not  an  uncommon 
condition.  About  6^  per  cent,  of  all  eyes  have  this  form 


262        REFRACTION  AND  HOW  TO  REFRACT. 

of  refraction.  This  is  a  combination  of  the  simple  hyper- 
opic  and  simple  myopic  astigmatisms,  with  their  axes  oppo- 
site or  at- right  angles  to  each  other,  as  a  rule. 

April  8th.     MR.  CROOK.     Ag°,  twenty-one  years.     Single.     Clerk. 

VI 
O.  D.  V.  =  -J(L.     p.  p.  =  14  cm.  with  type  0.75  D. 

VI 
O.  S.  V.  =  -j^£.     p.  p.  =  14  cm.  with  type  0.75  D. 

Add.,  20.     Abd.,  to. 

History  of  poor  sight  all  his  life,  but  thinks  it  was  better 
as  a  boy.  Has  frequent  frontotemporal  headaches,  which 
are  worse  after  using  eyes  at  any  prolonged  near-work. 
Father  has  good  sight,  but  his  mother  and  her  family  have 
all  been  near-sighted ;  has  one  aunt  that  developed  cata- 
racts. Has  been  to  several  "stores,"  but  could  not  get 
fitted  with  glasses  that  would  improve  his  vision. 

5.  P. — Face  broad  and  symmetric.  Long  interpupillary 
distance.  Irises  dark  in  color.  Pupils  large,  5  mm.  ; 
round.  Eyes  out  under  cover. 

Ophthalmoscope. — O.  D.,  media  clear.  Disc  vertically 
oval,  axis  105.  Macular  region  shows  changes.  Vessels 
at  105  best  seen  with  4-2,  and  at  right  angles  with  — 2. 
O.  S.,  same  conditions  found,  except  that  the  meridians  are 
at  75  degrees  and  165  degrees. 

OphtJialmometer. — O.  D.,  4  D.  axis  100.  O.  S.,  4  D. 
axis  75. 

Cobalt-blue  Glass. — O.  D.  violet  center,  blue  above  and 
below  in  meridian  of  105  and  red  at  the  sides  in  meridian 
of  15.  O.  S.,  violet  center,  blue  above  and  below  in 
meridian  of  75,  and  red  on  sides  at  axis  165.  (See  Figs. 
135  and  136.) 


Stenopeic  Slit. — O.  D.  axis   15  with   +2  sph.,  V.= 


VI 


VI 
IX 


at  axis  105  with  — 2  sph.,  V.  =  — .     O.  S.  axis   165  with 
4-2  sph.,  V.=r  -y^- ;  at  axis  75  with  — 2  sph.,  V.=  — . 


APPLIED    REFRACTION.  263 

Indirect  Method.  —  Each  eye  shows  a  lengthening  of 
the  vertical  meridian  as  the  condensing  lens  is  withdrawn 
from  the  eye,  and  at  the  same  time  the  horizontal  meridian 
grows  narrower.  As  the  lens  is  advanced  toward  the  eye 
the  vertical  meridian  grows  shorter  and  the  horizontal 
meridian  grows  broader. 

,/    The  astigmatic  chart  does  not  show  any  difference  in  the 
/shading   of   the   lines  ;    they  all   appear  about  the  same. 
Retinoscope  at  1  meter  distance  shows  myopia  in  the  verti- 
cal meridian  and  hyperopia  in  the  horizontal. 

R  .     Atropin  and  dark  glasses  for  refraction. 

April  loth  :  At  six  meters  from  test-card  and  point  of 
light.  O.  D.  and  O.  S.  V.  =  ^. 

L«A 

Cobalt-blue  glass  shows  the  same  as  at  first  visit. 

Retinoscope  at  the  distance  of  one  meter  shows  point  of 
reversal  in  O.  D.  at  axis  105  degrees  with  —  1.500.,  and 
axis  15  with  +3  D.  O.  S.,  axis  75  with  —  1.50  D.,  and 
axis  165  with  +3  D. 

Stcnopeic  Slit.  —  O.  D.,  axis  15  with  -\-2  sph.,  V.  = 
jNx!  ;  axis  105  with  —  2.50.  sph.,  V.  =  -^.  O.  S.,  axis 
165  with  +2  sph.,  V.  =  ^  ;  at  axis  75  with  —  2.50  sph., 

y  -  .-.XL 

'    ix  ; 

At  Trial-case.  —  O.  D,  —  2.50  cyl.  axis  15  degrees  O 
-f-2  cyl.  axis  105  degrees,  V.  =  [ls-  O.  S.,  —  2.50  cyl. 


axis  165  O  +2  cyl.  axis  75,  V.  = 


VIISS* 

Or, 

VI 

O.  D.,  —  2.50  sph.  O  4  4-  50  cyl.  axis  105,  V.  —  ylfsg. 


VI 
O.  S.,  —2.50  sph.  O  -(-4-5°  cyl.  axis    75,  V.  —  ^ss>. 

Or, 

VI 

O.  D.  -f-  2  sph.  O  —  4.50  cyl.  axis  15  degrees,  V.  =  yjjgg  -f- 

VI 

O,  S.    -j-2  sph.  O  —  4.50  cyl.  axis  165,  V.  =  V1ISS  -\-. 


264        REFRACTION  AND  HOW  TO  REFRACT. 

April  nth:  Same  results  as  April  loth.  Add.,  2O. 
Abd.,  8. 

For  MR.  CROOK. 

B.     O.  D.  -\-2  sph.  O  — 4.50  cyl.  axis  15    degrees. 

O.   S.  -f-2  sph.  O  — 4.50  cyl.  axis  165       " 
SlG. — For  constant  use. 

April  1 2th  :  Glasses  neutralize  ;  are  centered  and  adjusted. 

April  26th:  Near  point  10  cm.,  which  is  consistent  with 
age  of  patient. 

Considerations.  —  The  ophthalmoscope,  retinoscope, 
cobalt-blue  glass,  indirect  method,  and  stenopeic  slit  were 
direct  guides  to  the  character  of  the  refractive  error. 
Emphasis  is  placed  upon  these  different  methods,  as  so  many 
beginners  in  ophthalmology  have  a  fear  or  dread  of  the  re- 
sult in  refracting  cases  of  mixed  astigmatism. 

The  stenopeic  slit  shows  a  difference  of  4.50  in  the  two 
principal  meridians  ;  bearing  this  fact  in  mind,  if  a  — 4.50 
cylinder  at  axis  1 5  be  placed  before  the  right  eye,  then  all 
meridians  would  be  made  equally  hyperopic  2  D.  Com- 
bining -f-  2  sph.  with  the  — 4. 50  cylinder  at  axis  1 5  in  the 
right  eye  or  at  axis  165  in  the  left,  the  refraction  would  be 
corrected. 

Or,  if  a  +4.50  cylinder  at  axis  105  be  placed  before  the 
right  eye,  then  all  meridians  would  be  made  myopic 
2.50  D.  Combining  a  — 2.50  sph.  with  this  +4.50  cylin- 
der at  axis  105  in  the  right  eye  or  at  axis  75  in  the  left  eye, 
the  refraction  would  be  corrected. 

For  a  further  consideration  of  combination  of  lenses  see 
page  51. 

The  rule  for  ordering  cylinders  is  the  same  in  mixed 
astigmatism  as  in  other  cylindric  corrections — without 
change. 

SUMMARY. — The  character  of  the  astigmatism   is  regu- 


APPLIED    REFRACTION.  265 

lar,  symmetric,  and  with  the  rule.     The  near  point  returns 
to  the  normal  for  the  age. ,.  Eyes  with  such  errors  do  not, 


VI 


as  a  rule,  obtain  a  visual  acuity  of  —,  for  the  reason  that 


changes  have  taken  place  in  the  eye-ground,  especially  at 
the  macula.  ' 

CASE  VIII. — Irregular  Astigmatism. — 

April  ad.     MARY  SMILES.     Age,  ten  years.     Scholar. 

VI 
O.  D.  V.  =  xxx   slowly.     No  p.  p.  obtained. 

VI 
O.  S.  V.  =  LX~.     No  p.  p.  obtained. 

History  of  poor  sight  ever  since  an  attack  of  measles 
when  two  years  of  age,  at  which  time  was  kept  in  a  dark 
room  for  six  weeks.  Eyes  were  never  strong  afterward  ; 
always  very  sensitive  to  light.  Child  was  sent  home  from 
school  with  a  note  from  the  teacher  :  "  Mary  is  near- 
sighted and  should  see  a  doctor." 

6".  P. — Eyelids  appear  normal.  Excessive  epiphora. 
Corneas  nebulated,  especially  O.  S.,  which  has  a  decided 
leukoma  at  the  pole.  Anterior  chambers  of  normal  depth. 
Pupils  3  mm.,  round.  Corneal  reflex  very  irregular. 

Ophthalmoscope. — No  view  obtained  of  the  eye-ground 
through  the  small  pupils  on  account  of  corneal  opacities. 
Homatropin  mydriasis  shows  O.  D.  cornea  faintly  nebu- 
lated in  scattered  areas  ;  rest  of  media  clear.  Nerve  small 
and  round.  Vessels  at  axis  35  degrees  best  seen  with  -f-2 
D.  O.  S.,  there  is  a  3  mm.  area  of  opacity  at  the  pole  of  the 
cornea  ;  no  clear  view  of  the  eye-ground.  Indirect  method 
shows  a  small  nerve  and  refraction  hyperopic. 

R .     Atropin  and  dark  glasses  for  refraction. 

April  22d  :  Retinoscope  at  I  meter  shows  band  of  light 
at   axis    35,   indicating   hyperopia.      Other  meridians   very 
irregular.     O.  S.,  nothing  definite  made  out. 
23 


266        REFRACTION  AND  HOW  TO  REFRACT. 

Placido's  disc  shows  irregular,  distorted  circles. 

With  Put-hole  Disc.—Q.  D.  V.  =  Yv'-.    O.  S.  V.  =  ^-. 

A  A  1  ,  \ 

With  Stcnopeic  Slit.  —  Axis  45  degrees  before  O.  D.  and 
with  +2.25  S.,  V.  =  ~  ?  ?.  O.  S.,  can  not  improve 
vision  with  any  glass. 

At  Trial-case.  —  O.  D.,  +2.00  cyl.  axis  145  =  ^??. 
O.  S.,  no  glass  accepted. 

April  2$d  :  At  trial-case  O.  D.,  +1.75  cyl.  axis  35  =  ~. 

For  MARY  SMILES. 

R.     O.  D.,  —  0.25  sph.  O  +1-75  cyl.  axis  35  degrees. 
O         O.  S.,  plane  glass. 
SIG.  —  Constant  ^use. 

Considerations.  —  This  case  shows  the  advantage  of  the 
stenopeic  slit  and  the  use  of  the  pin-hole  disc.  The  near 
point  could  not  be  obtained  on  account  of  the  poor  visual 
qualities  and  the  child's  inability  to  appreciate  what  was 
wanted. 

CASE  IX.  —  Tonic  Cramp  or  Spasm  of  the  Accommo- 
dation. — 

MRS.  L.     Age,  twenty-four  years. 

VI 
O.  D.  V.  =  xxv??.    p.  p.  =9  cm.     (?)  Add.,  24  degrees.     Abd.,  6 

degrees.     Esophoria,  4  degrees. 

VI 
O.  S.  V.  =  "xxv  ^  ***     P-  P-  —  9  cm-     (**)  ^°  vertical  deviation. 


History  of  having  had  glasses  changed  on  three  different 
occasions  during  the  past  year.  Drops  were  used  each 
time,  and  the  three  prescriptions  were  all  different.  Glasses 
were  always  satisfactory  for  the  first  wreek,  but  after  this 
time  she  was  always  able  to  see  better  at  a  distance  with- 
out them.  Has  pains  in  her  eyes  and  all  over  the  head 
whenever  she  attempts  to  use  the  eyes  with  or  without  any 
glasses.  Headaches  nearly  set  her  "  wild"  if  she  tries  to 


AITI.IKI)    KK1  KACT1ON.  267 

concentrate  her  vision  on  a  distant  or  near  object.  Has  not 
been  able  to  read  or  write  or  sew  for  the  past  two  years. 
Has  been  under  the  care  of  the  gynecologist  and  neurologist, 
and  they  each  pronounce  her  physical  condition  as  normal. 
The  neurologist  suggests  a  diagnosis  of  "  hysteria." 
Patient  sleeps  well  and  has  a  good  appetite,  but  will  suffer 
from  nausea  and  vomiting  if  she  uses  her  eyes  for  any  length 
of  time.  Patient  has  been  married  five  years.  Has  one  living 
child.  No  miscarriages.  Is  apparently  in  the  very  best 
of  health,  and  is  provoked  with  her  apparent  good  health 
as  not  being  consistent  with  her  suffering,  and  hence  she 
does  not  receive  any  sympathy  from  her  family  or  her 
friends. 

5.  P. — No  external  manifestations  of  any  ocular  irregu- 
larity. 

Manifest  Refraction. — 

VI 

O.  D.,  — 0.50  S.  O  +1.00  cyl.  axis  90  degrees  =  -yj-. 

O.  S.,  the  same  as  O.  D. 

Ophthalmoscope. — O.  D.  and  O.  S.,  media  clear.  Discs 
vertically  oval,  eye -grounds  "woolly."  Accommodation 
very  active.  Shot  silk  retina.  Refraction  is  compound 
hyperopic  astigmatism. 

Cobalt-blue  glass  shows  a  red  center  and  broad  blue  halo. 
(Patient  is  certainly  accommodating.) 

& .     Atropin  and  dark  glasses  for  refraction. 

Static  Refraction. — 

VI 

O.  D.  +1.50  S.  =  +1.75  cyl.  axis  90  degrees  =  -y-. 

O.  S.  +1.50  S.  =  -j-l.75  cyl.  axis  90  degrees  =  -XL. 

Patient  states  that  her  "  pains  and  headaches  all  disap- 
peared after  using  the  drops  for  the  third  time." 


268        REFRACTION  AND  HOW  TO  REFRACT. 

Refraction  repeated  on  three  different  occasions,  and  the 
following  prescription  given  : 

For  MRS.  L. 

R.     O.  D.,  +1.25  S.  O  +1-75  cyl.  axis  90  degrees. 
O.   S.,  +1.25  S.  O  +1-75  cyl.  axis  90  degrees. 

Glasses  properly  centered,  and  accurately  adjusted. 
r      After  ten  days  patient  returns  with  the  statement  that 
her  pains  and  aches  have  recurred  as  before,  and  that  she 
can  see  better  at  a  distance  without   her  glasses.     With 
correction,  each  eye  sees  ^~,  and  with  both  eyes  can  see 
Add.,   20  degrees,  and  abd.,  8  degrees.     No  verti- 


cal deviation.     Has  3  of  esophoria  at  6  meters. 

Atropin  -£$  of  a  grain  to  the  ounce. 
SlG. — To  use  one  drop  in  each  eye  each  morning  and  noon. 

To  wear  a  pair  of  dark  glasses  with  her  prescription  glasses 
when  exposed  to  any  bright  light.  N^LJxL-attempt ..any 
near-work.  This  treatment  was  continued,  off  and  on,  for 
six  months.  Patient  was  always  free  from  ocular  pain  and 
headache  as  long  as  the  atropin  was  being  used,  but  as 
soon  as  the  ciliary  muscle  commenced  to  contract,  then  the 
pains  would  return  with  all  their  former  severity.  This 
patient  eventually  recovered  by  using  her  distant  correc- 
tion with  a  pair  of  plus  2  spheres  as  hook -fronts  for  any 
near-work. 

CASE  X. — Exophoria. — 

Miss.  V.  B.  D.     Age,  twenty-two  years. 

VI 
O.  D.  V.  =  -yj-.     p.  p.  =  0.50  D.,  type  at  n  cm. 

VI 
O.  S.  V.  =  -yy.     p.  p.  =  0.50  D.,  type  at  II  cm. 

Add.  and  abd.,  12  degrees.     Exophoria  =  4. 

History  of  seeing  double  several  times  a  day.  Friends 
and  members  of  her  family  have  told  her  she  was  "  squint- 


' 


APPL1KD    K  INFRACTION.  269 

ing."  Always  returns  home  with  a  severe  occipital  head- 
ache after  going  shopping  or  to  any  place  of  amusement. 
Has  headache  when  using  her  eyes,  but  it  soon  passes 
away  after  resting  the  eyes. 

S.  P.  —  Eyes  markedly  out  under  cover.  Irises  react 
promptly  to  light,  accommodation,  and  convergence. 
Fixation  test  shows  the  right  eye  divergent. 

Ophthalmoscope.  —  O.  D.  and  O.  S.  No  apparent  changes, 
and  refraction  almost  emmctropic  ;  some  small  amount  of 
hyperopia  and  astigmatism. 

R  •     Atropin  and  dark  glasses  for  rest  and  careful  refraction. 

Static  refraction,  after  several  repetitions,  O.  D.  and  O. 
S.,  +0.50  S.  Cl  4  0.37  cyl.  axis  90  degrees  =  -~.  And 
this  is  ordered,  less  0.25. 

With  this  correction  carefully  centered,  add.  =14  degrees 
and  abd.  =  1  2  degrees,  with  3  of  exophoria  at  6  meters  and 
7  degrees  of  exophoria  at  1  3  inches.  This  patient  was  given 
prism  exercises  for  more  than  two  months,  and,  finally, 
after  the  adduction  reached  30  degrees  and  abduction  was 
10  degrees  and  3  degrees  of  esophoria  were  obtained,  the 
prism  exercises  were  stopped,  and  patient  told  to  report 
promptly  if  any  disconifort  arose  at  any  time.  To  wear  .  /  '^IjJL 
her  glasses  constantly,  ®T&I  1 

H 


MR.  ALBERT  S.     Age,  twenty-nine  years.     In  general  business. 

VI 
O.  D.  V.  =  LX".     p.  p.  type  0.50  D.  =  20  cm. 

VI 
O.  S.  V.  =  xxx  .     p.  p.  type  0.75  D.  =  30  cm. 

Add.,  10  degrees.     Abd.,  6  degrees.     Left  Hy.,  2  degrees. 

History.  —  Has  had  three  pairs  of  glasses  ordered,  with 
"  drops,"  during  the  past  eighteen  months.  Has  never 
had  any  but  the  very  slightest  relief  from  ocular  pains  and 


2/O  REFRACTION     AND     HOW    TO     REFRACT. 

frontal  headaches,  which  have  been  almost  constant  for  the 
past  four  years  or  more.  On  account  of  the  ocular  dis- 
comfort and  headaches,  the  patient  has  given  up  all  at- 
tempts to  read  for  more  than  fifteen  minutes  at  a  time 
Patient  states  that  if  he  uses  his  eyes  for  more  than  thu 
length  of  time  they  become  bloodshot  and  very  tendci 
to  the  touch.  General  health  of  patient  is  excellent  ;  ha.1 
a  good  appetite  and  sleeps  well.  Docs  not  use  tobacco  01 
liquor  of  any  kind. 

5.  P. — Face  symmetric.  Nose  very  prominent.  Inter 
pupillary  distance,  62  mm. 

OphtJialmoscope. — O.  D.,  nerve-head  over  capillary.  No 
swollen.  Accommodation  very  active.  Eye-ground  "fluffy.' 
Refraction  is  that  of  compound  hyperopia.  O.  S.,  sam< 
general  conditions,  but  the  nerve  is  vertically  oval  and  th< 
refraction  is  compound  hyperopic  astigmatism. 

R .     Atropin  and  dark  glasses  for  refraction. 

Static  Refraction. — 

O.  D.,  4-2.00  S.  O  4  I -oo  cyl.  axis    75  degrees       ^.j  ~\  ^  ° 

O.   S.,  +1.25  S.  C  +3.00  cyl.  axis  105  degrees  =-^,| -???  j  h>ln>r- 

K.     O.  D.,  -fl-75  S.  O  +l.oo  cyl.  axis    75  degrees  O  ^  A  Dase  UP- 

O.   S.,  -f  l-ooS.  O  +3-OOcyl.  axis  105  degrees  O  ^/^  basedowi 
SlG. — For  constant  use. 

This  patient  was  rrot  made  comfortable  until  he  was  give 
five-grain  doses  of  the  bromid  of  potash  three  times  a  da 
for  four  weeks.  Is  now  able  to  use  his  eyes  without  th 
least  discomfort. 


, 


x 


jf    ^ 


MM. 

y 


1 


I  /  • 


4-js- 


CHAPTER  XL 

PRESBYOPIA.— APHAKIA.— ANISOMETROPIA. 
—SPECTACLES. 

Presbyopia. — The  word  presbyopia  (from  the  Greek, 
TT/^'VMT,  "old"  ;  "V',  "eye")  literally  means  old  sight,  and 
patients  at  the  age  of  forty-five  or  more  years  are  univer- 
sally recognized  as  presbyopes,  and  the  condition  of  their 
eyes  as  presbyopia  There  is  no  exact  age  limit  as  to  when 
presbyopia  shall  begin,  the  advent  of  presbyopia  being  con- 
trolled by  the  character  of  the  ametropia  and  physical  con- 
dition of  the  eyes  themselves.  Presbyopia  may  be  described 
in  several  different  ways,  according  to  the  cause — /.  e., 

1.  Old  sight. 

2.  The  condition  of  the  eyes  in  which  the  punctum  proxi- 
mum  has  receded  to  such  a  distance  that  near  vision  (close 
work)  is  impossible  without  the  aid  of  convex  lenses. 

3.  The  condition  of  the  eye  in  which  the  lens  fibers  have 
become  more  or  less  sclerotic,  and,  as  a  consequence,  the 
lens  loses  some  of  its  inherent   quality  of  becoming  more 
convex  during  contraction  of  the  ciliary  muscle. 

4.  The  condition  of  the  eye  in  which  the  power  of  the 
ciliary  muscle  has  become  weakened. 

5.  The  condition  of  the  eye  in  which  the  power  of  ac- 
commodation is  diminished  at  the  same  time  that  the  lens 
fibers  become  sclerotic. 

6.  The  condition  of  the  eye  in  which  two  different  refrac- 
tions (not  necessarily  two  pairs  of  glasses)  are  required,  one 
for  distance  and  one  for  near  vision. 

271 


272 


REFRACTION  AND  HOW  TO  REFRACT. 


7.  The  condition  of  the  eye  in  which  one  pair  of  glasses 
will  not  answer  for  distant  and  also  for  near  vision. 

8.  Presbyopia  may  be  described  as  the  condition  in  which 
nature  has  instilled  a  slowly  acting  but  permanent  cyclo- 
plegic  (the  term  cycloplegic  is  used  here  in  a  general  sense). 

Causes  of  Presbyopia. —  I.  Age. — It  is  a  well-estab- 
lished fact  that  in  childhood  the  center  of  the  lens  begins 
to  harden,  becomes  sclerotic  or  sclerosed,  to  form  a  nucleus  ; 
and  this  process  continuing,  eventuates  in  complete  sclerosis 
at  sixty  or  seventy-five  years.  The  term  sclerosis  must  not 
be  confounded  with  opacity. 

2.  Disease. — Ordinarily,  presbyopia,  as  applied  to  the 
lens,  should  be  recognized  as  a  physiologic  process,  as  a 
penalty  for  growing  old,  though  it  is  a  condition  which 
may  be  hastened  by  disease.  Any  disease,  therefore,  which 
will  cause  the  nutrition  of  the  lens  to  suffer  must  event- 
ually interfere  with  its  ability  to  become  more  convex  during 
accommodation.  The  most  common  ailments  that  tend  to 
this  result  are  rheumatism,  gout,  Bright's  disease,  diabetes, 
lithiasis,  la  grippe,  etc.  Any  disease  which  will  weaken  the 
ciliary  muscle  will  produce  presbyopic  symptoms. 

Presbyopic  Near  Points. — The  near  point  and  power  of 
accommodation  in  a  healthy  emmetropic  eye,  or  a  healthy 
eye  made  emmetropic  by  the  addition  of  correcting  lenses, 
is  as  follows  for  certain  ages  : 


POWER  OF 

AGE.                        H     NEAR  POINT. 

ACCOMMODATION. 

40  years,     .    .   £j^j   .    .     22cm.             4 

5° 

diopters. 

45       ' 

.  .   .  u'O  .    28  « 
ri 

3 

50 

5°      ' 

.    .    .    /  (o.    .    40    ' 

2 

50 

55      ' 

...**.    55   ' 

I 

75° 

r  2.00 

60      ' 

.    .    .    .t?  i  loo   « 

I 

00 

65      ' 

.    .    .  JFp1.  133    ' 

0 

75 

70      ' 

.    .    .  |*0(  .  400   • 

o 

25 

7C        ' 

oo   ' 

oo 

PRESBYOPIA.  2/3 

Ordinarily,  the  average  adult  holds  a  newspaper  or  book 
at  about  33  cm.  (13  inches)  from  his  eyes  when  reading  ; 
and  if  he  is  forty  years  of  age  and  emmetropic,  or  is  made 
emmetropic  with  glasses,  he  would  be  using  3  D.  of  his 
normal  4.50  of  accommodation,  which  would  leave  a  reserve 
power  of  1.50  D.;  and  in  this  condition,  other  things  being 
equal,  he  can  maintain  a  reading  distance  with  comfort.  In 
fact,  he  could,  by  using  all  of  his  4.50  D.  of  accommoda- 
tion, see  objects  as  close  as  22  cm.,  but  not  for  any  great 
length  of  time,  as  the  ciliary  muscle  would  soon  relax. 

This  same  patient  at  forty-five  or  forty -six  years  of  age  will 
have  lost  i.oo  or  1.50  D.  of  his  accommodation,  and  now 
has  only  about  3  or  3.50  left  ;  and  if  he  uses  all  of  it  at  a 
working  distance  of  33  cm.,  the  ciliary  muscle  soon  yields. 
In  fact,  the  ciliary  muscle  can  not  be  held  in  such  a  state  of 
tension  without  causing  all  sorts  of  pains  and  aches  and 
reflex  disturbances  ;  and  the  ciliary  effort  relaxing  suddenly, 
the  near  vision  blurs,  and  the  work  or  reading  or  sewing 
must  be  put  at  a  greater  distance  to  obtain  relief,  or  else 
the  effort  must  be  abandoned. 

Symptoms  of  Presbyopia. — The  principal  symptom  is 
that  which  indicates  a  recession  of  the  punctum  proximum  ; 
the  patient  stating  that  there  is  an  inability  to  maintain 
the  former  reading,  writing,  or  sewing  distance,  and  that  all 
near-work  must  be  held  at  a  greater  distance  than  formerly. 
Symptoms  of  accommodative  strain  may  be  present  if  the 
patient  endeavors  to  force  the  accommodation  to  its 
maximum. 

Diagnosis. — The  age  of  the  patient  and  the  history  of 
having  to  hold  reading  matter  at  an  uncomfortable  distance  ; 
or  a  history  of  good  distant  vision  and  an  inability  to  retain 
clear  near  vision — small  objects,  to  be  seen,  must  be  held 
far  away  or  "  at  arm's  length." 


274 


REFRACTION  AND  HOW  TO  REFRACT. 


^^ 

ft 


Correction  of  Presbyopia.  —  The  presbyopic  state  repre- 
sents a  class  of  patients  for  whom  glasses  may  be  pre- 
scribed by  the  manifest  refraction,  although  there  are 
exceptional  cases  in  which  a  quick  cycloplegic  will  be  nec- 
essary when  an  amount  of  astigmatism  or  cylinder  axis  is 
uncertain. 

r  a  working,  reading,  writing,  or  sewing  distance  of  33 
cm.  (13  inches),  the  writer  makes  it  a  rule  to  add  to  the  dis- 
tance correction  at  forty-  five  years  of  age  a  +  I  sphere  ;  at 
fifty  years  of  age,  a  -f-  2  sphere  ;  at  fifty-five  years  of  age, 
a  +2.50  sphere  ;  and  for  sixty  or  more  years,  a  +3  sphere. 


The  following  table  for  emmetropic  eyes  shows  these  addi- 
tions for  the  different  years,  and  also  the  near  and  far  points 


/  /A 


feTJ 

tt*K/ 


with  these  additions  as  well  as  the  range  of  accommo- 
dation or  "play  "  between  the  near  and  far  points.  It  will 
be  observed  that  the  range  of  78  cm.  at  forty-five  years 
rapidly  diminishes  in  the  succeeding  years,  until  at  sixty 
there  is  a  play  of  only  about  3  inches,  and  at  seventy  the 
range  is  practically  gone.  , 


Iioo  cm. 
50 
40 
33 

:    /  33 
/  33 

/  / 33 

'S.-f-f  C4*S    A+~a£**  ^ 

a  patient  is  fifty  years  of  age  does  not  signify 
at  he  will  be  able  to  read  at  33  cm.  with  a  pair  of  -\-2 
spheres,  or  because  he  is  sixty  years  of  age  that  he  can  use 
his  eyes  at  33  cm.  comfortably  with  a  pair  of  +3  spheres  j 
on  the  contrary,  this  rule  that  the  writer  has  given  applie 
only  to  cases  of  emmetropia.  It  often  happens  that  pres- 
byopic  patients  state  that  they  do  not  want  glasses  for  dis-  I  ~ 


. 

YEARS. 

ADD. 

NEAR  POINT. 

45 

+  1,00 

22  cm. 

So 

^^^+2.00 

22    " 

55 

+  2.50 

23    " 

60 

-1-3.00 

25    " 

65 

-f3-oo 

27    " 

fl:LV 

>        7o 

+3.00 

30    " 

,.     JW 

2.          I)/) 

75 

-h3-oo 

33    " 

IMA*  f  H  rrfa 

M<^A^rf  * 

lA/fl 

NT.            RANGE 

i.                78  en 

;.    ^, 

28    • 

17    ' 

8    ' 

6    • 

3    ' 

o   ' 

/ 

.0.? 


/", 

/~~ 


PRESBYOPIA.  275 

tance  ;  that  they  do  not  need  them  ;  that  all  they  wish  is  a 
pair  of  glasses  to  use  at  near-work,  reading,  ete.  When  the 
vision  is  taken  in  such  cases,  it  may  be  found  to  be  — —  or 


vi 


VI 


approximating     .-  ;  but  the  young  ophthalmologist   must 
not  be  thrown  off  his  guard  by  this  record,  as  it  has  already 


VI 


been  stated  that  a  vision  of  VI  does  not  by  any  manner 
of  means  prove  the  existence  of  emmetropia.  Let  the  sur- 
geon make  it  a  constant  rule  in  every  case  of  presbyopia  to 
always  carefully  estimate  the  amount  of  the  distance  ametropia 
^/^  first,  no  matter  how  ^veak  or  what  its  form  (sphere  or  cylinder)  ; 
and  to  the  result  thus  obtained,  add  the  plus  sphere  which  will 
be  required  for  the  ivorking  distance  or  point  at  which  tJie 
patient  wishes  to  see  clearly.  £  /&t-*~^-yiS  '•? 

Illustrative  Cases.  —  CASK  I.  —  Accepts  +0.50  sph.  for 
distance.  At  forty-five  years  this  case  would  require 
-f  1.  50  sph.  for  reading  at  a  distance  of  33  cm.;  at  fifty  years, 
+  2.50  sph.;  at  fifty-five  years,  +3  sph.;  and  at  sixty  or 
more  years,  +3.50  sph.  Only  one  pair  of  glasses  is  neces- 


CASE  II.  —  Accepts  -(-2  sph.  for  distance;  at  forty-five 
years  these  eyes  would  require  +3  sph.;  at  fifty  years  they 
would  require  -[-4.50  sph.;  and  at  sixty  or  more  years  they 
would  require  -j~5  sPn-  Two  pairs  of  glasses  would  be 
indicated  in  this  case. 

CASE  III.  —  Accepts  —  i.oo  sph.  for  distance  ;  at  forty- 

five  years  this  patient  could   read   without  any  glasses,  as 

—  I  for  distance  would  be  neutralized  by  the  -f  I   required 

for  reading.     At   fifty  years,  however,   the   patient   would 

require  a  -f  I  sphere  for  near,  and  at  fifty  -five  a  -f-  1.50,  and 

at  sixty  years  a  -f-2  sphere.      Case  II  required  two  correc- 

tions, one  for  distance  and  one  for  near  ;  and  the  same  may 

'besaid  about  Case  III  ;  but  in  this  latter  instance  there  was 

a  time  at  forty-five  years  when  there  was  no  necessity  for 


276 


REFRACTION  AND  HOW  TO  REFRACT. 


glasses  for  the  near-work,  as  the  patient's  eyes  were  in  a 
suitable  condition  of  refraction  to  read  without  them. 

CASE  IV. — Accepts  — 3  sph.  for  distance.  At  forty-five 
years  would  require  — 2  sph.  for  reading  ;  at  fifty  years 
would  require  — I  sph.  for  reading  ;  and  at  sixty  years 
can  read  without  any  glasses.  Such  a  patient  says  he  has 
gotten  his  "  second  sight." 

CASE  V. — Accepts  -)  0.50  cylinder  axis  1 80  for  distance 
and  requires  the  usual  additional  spheres  for  the  increasing 
years  for  his  reading  distance. 

CASE  VI. — Accepts  -fi.oo  sph.  O  -j-i-OO  Cyl.  axis  180 
for  distance  and  requires  the  spheric  additions  as  the  years 
increase.  Two  pairs  of  glasses  should  be  prescribed. 

CASE  VII. — Accepts  — i  cyl.  axis  90  for  distance,  and 
requires  -(-  I  cyl.  axis  180  to  read  with  at  forty-five  years 
of  age  ;  at  fifty  years  he  requires  -f- 1  sph.  O  -f  I  cyl. 
axis  1 80  ;  and  at  sixty  years  requires  +2  sph.  O  + 1  cyl. 
axis  1 80.  At  forty-five  years  of  age  this  patient  is  com- 
monly spoken  of  as  having  simple  myopic  astigmatism  for 
distance  (against  the  rule)  and  simple  hyperopic  astigmatism 
for  near  (against  the  rule  also)  ;  two  pairs  of  glasses  are  in- 
dicated throughout  life. 

CASE  VIII. — Accepts  — i.oo  sph.  O  — 1.50  cyl.  axis 
1 80  for  distance,  and  at  forty-five  years  will  need  — 1.50  cyl. 
axis  1 80  for  reading  ;  at  fifty  years  will  require  -(-  i.oo  sph. 
O — 1.50  cyl.,  axis  180  degrees;  at  sixty  years,  +0.50 
sph.  O  -f- 1.50  cyl.  axis 

Two  pairs  of  glasses 

At  forty-five  years  this  patient  has  a  compound  myopic 
correction  for  distance  and  simple  myopic  astigmatism  for 
near  ;  at  fifty  years  the  correction  for  near  is  that  of  crossed 
cylinders  (mixed  astigmatism)  ;  and  at  sixty  years  the  near 
correction  is  that  for  compound  hyperopic  astigmatism. 


\s  *•-*.-*  — /  r* 

5  QQ  degrees.  *-fTC  -/$"//*«  ~t~2 

rnM$r£<~     .  *i&u*&i.9T'r* 

s  should  be  used  rTjrniignmiK  lif        -^  - 


v< 
<*Vj 


PRESBYOPIA.  277 

CASE  IX. — Accepts — i.oosph.  O  -f  2  cyl.  axis  90  for  dis- 
tance (mixed  astigmatism)  ;  at  forty-five  years,  -\-2  cyl.  axis 
90  is  required  for  reading  ;  at  fifty  years,  -)-i.oo  sph.  O 
+  2.00  cyl.  axis  90.  Tu^cTpairs  of  glasses  are  required.  At 
forty-five  years  the  distance  correction  is  for  mixed  astig- 
matism and  the  reading  correction  is  for  simple  hyperopic 
astigmatism. 

CASE  X. — Accepts  — 2.00  cyl.  axis  180  for  distance  ; 
at  forty-five  years  requires  a  mixed  astigmatism  correction 
for  near;  at  fifty  years,  a  simple  hyperopic  correction * 
and  a  compound  hyperopic  correction  at  sixty  years.  - 

In  the  above  illustrative  cases  the  working  distance  has 
been  calculated  at  33  cm.,  or  13  inches;  but  as  some 
patients  use  their  eyes  at  a  greater  or  less  distance  than 
this,  the  additional  convex  lenses  must  be  calculated  accord- 
ingly. For  instance,  the  weaver  at  fifty-five  years  of 
age  who  requires  -f  2  spheres  for  distance  could  not  see  to 
weave  at  50  cm.  ;  if  -(-2.50  spheres  were  added  to  his  dis- 
tance correction,  all  he  needs  is  -(-3  for  his  working  dis- 
tance. Or  the  diamond  cutter  who  wishes  glasses  to  see 
his  work  at  8  inches,  if  he  accepted  — i.oo  sph.  for  distance, 
lie  would  require  +2  sph.  at  forty-five  years  of  age. 

In  conclusion,  there  are  three  facts  in  the  refraction  of 
presbyopic  patients  that  should  receive  attention  : 

i.  Many  accept  a  weak  plus  cylinder  (  +  0.50  at  axis 
1 80)  against  the  rule.  This  is  presumptive  evidence  that 
the  astigmatism  is  acquired,  is  lenticular,  and  is  due  to  the 
sclerotic  changes  previously  mentioned.  The  only  positive 
way  to  prove  this  fact  is  by  the  retinoscope,  and  by  the  ab- 
sence of  corneal  astigmatism  with  the  ophthalmometer. 
If  the  case  has  been  previously  refracted  by  the  same  sur- 
geon, his  record  will  also  confirm  this  extremely  interesting 
occurrence.  According  to  able  authorities,  hyperopic  eyes 


2/8        REFRACTION  AND  HOW  TO  REFRACT. 

become  more  hyperopic  after  the  age  of  seventy  years, 
and  emmetropic  eyes  may  become  hyperopic,  and  myopic 
eyes  less  myopic,  from  the  same  sclerotic  or  shrinking  pro- 
cess which  takes  place  in  all  the  ocular  tissues  as  a  result 
of  senility.  The  method  of  correction  by  glasses,  however, 
is  just  the  same,  and  that  is  to  correct  the  distant  vision 
first  and  then  add  the  near  correction. 

2.  An  attack  of   glaucoma  may  precipitate  presbyopic 
symptoms,  so  that  when  a  presbyopic  patient  asks  for  fre- 
quent  changes  in  his  corrections,  this  complication  should 
be  borne  in  mind. 

3.  The  swelling  of  the  lens  which  occasionally  precedes 
the  formation  of  some  forms  of  cataract  should  be  remem- 
bered  when  the  patient  develops  symptoms  of  myopia — /.  i\, 
a  reduction  in  the  strength  of  convex  glasses.  (_ 

Aphakia  (d,  priv.  ;    <pax6<;   "  lentil ")  literally  means    an 
eye  "without  a  lens."     (See  Fig.  174.)    An  eye  which  has 

had  its  lens  dislocated 
has  been  erroneously 
spoken  of  as  aphakic. 
The  absence  of  the  lens 
means  a  total  absence 
of  all  accommodation, 

FlG  I7  no  matter  what  the  age 

of  the  patient  may  be. 

Causes. — Aphakia  may  be  congenital,  but  in  most  cases 
is  the  result  of  removing  the  lens  by  operation. 

Diagnosis. — Aphakia  maybe  diagnosed  by  inspection — 

i.  e.,   corneal  scar,  depth   of  anterior  chamber,    tremulous 

iris,  coloboma  of  the  iris,  opaque  capsule  whole  or  in  part, 

->  erect  corneal  image,  with  absence  of  lenticular  images,  and 

by  the  patient's  history. 

The  ametropia  of  an  aphakic  eye  depends  in  great  part 


HETEROMETROPIA. 


2/9 


upon  the  previous  refractive  condition  of  the  eye,  and  also 
upon  the  kind  of  operation  that  was  performed  for  the  re- 
moval of  the  lens.  It  has  been  calculated  that  an  eye,  to 
be  emmetropic  after  the  removal  of  its  lens,  would  have  to 
be  myopic  at  least  twelve  diopters.  If  this  is  always  true, 
then  the  correcting  lens  which  is  selected  by  an  aphakic  eye 
is  a  guide  to  its  former  ametropia.  An  eye  which  selects  a 
weak  plus  sphere  would,  therefore,  have  been  myopic  before 
the  operation  ;  and  if  about  a  +  12  S.,  its  previous  refrac- 
tion approximated  emmetropia  ;  if  a  plus  sphere  stronger 
than  1 2,  then  the  previous  refraction  was  very  likely  hyper- 
opia. 

An  eye  which   has  had   its   lens  removed   by  absorption/ 
(needling)  is   not   likely  to   be   astigmatic  ;  whereas,  when 
the    lens    has    been    removed    by  extraction,    astigmatism)  , 
against  the  rule  of  one  or  more  diopters  almost  invariable/—. 
results,  and  the  axis  of  the  correcting  cylinder   generally 
coincides  with  the  points  of  puncture  and  countcrpuncture 
in  the   cornea.       If  a  patient    had  2  or  3   D.   of  myopic 
astigmatism  with  the  rule,  this  would  be  neutralized  by  the 
corneal  section. 

Correction  of  Aphakia. — As  in  presbyopia,  two  correc-V^. 
tions  are  necessary — one  for  distance  and   one  for   noaxs^ 
Astigmatism  must  always  be  looked  for  and  carefully  cor- 
rected, especially  if  the  lens  has  been  removed  by  extrac- 
tion. 

CASE  I. — 

VI 

O.  D.,     48-00  sph.  ^  +3-OO  cyl.  axis  10  degrees.     V.  =    x  • 

O.  D.,  4  n.oo  sph.  C;  4  3-°°  c>'-  ax's  IO  degrees  =  reading  at  33  cm. 

This  patient  was  presumably  myopic  before  operation. 
Heterometropia  (£",""?,   "different";   !^r/mvt   "a  meas- 
ure" ;  &<}',  "the  eye")  literally  means  that  the  ametropia  of 


28O        REFRACTION  AND  HOW  TO  REFRACT. 

the  two  eyes  is  different  in  character;  examples,  O.  D.  -f-  i  D. 
and  O.  S.  — I  D.,  or  O.  D.  +3  cyl.  axis  90°  and  O.  S.  — 3 
cyl.  axis  180°,  or  O.  D.  — 5  D.  and  O.  S.  — 5  cyl.  axis 
1 80°,  etc. 

Anisometropia  («W« ?,  "unequal";  /j-l-pnv>  "a  measure"; 
&<}>,  "the  eye")  literally  means  that  the  ametropia  of  the 
two  eyes  is  the  same  in  character  but  of  unequal  amount. 
Examples,  O.  D.  +2  D.  and  O.  S.  +6  D.,  or  O.  D.  —0.50 
D.  and  O.  S.  — 5  D.,  or  O.  D.  +3  cyl.  axis  90°  and  O.  S. 
-{-6  cyl.  axis  90°,  etc.  This  condition  may  be  slight  or 
one  of  the  most  extreme  conditions  imaginable. 
^T'or  instance,  if  both  eyes  have  compound  hyperopic 
astigmatism,  they  are  not  considered  as  heterometropic,  even 
if  the  sphere  and  cylinder  are  of  different  strength  in  the 
two  eyes.  .  Bearing  this  distinction  in  mind,  the  percentages 
already  given  for  myopia,  hyperopia,  the  different  astigma- 
tisms, etc.,  have  been  calculated  accordingly,  that  for 
j^heterometropia  being  about  thirteen  per  cent. 

Causes. — Usually  the  condition  is  congenital,  or  it  may 
be  acquired. 

Difficulties. — Two  difficulties  are  encountered  when  or- 
dering glasses  for  cases  of  anisometropia  or  heterometropia : 
(i)  The  lens  for  one  eye  may  be  concave  and  that  for  the 
other  may  be  convex,  or  both  eyes  may  require  a  convex 
or  both  may  require  a  concave  lens,  but  one  very  much 
stronger  than  the  other;  under  these  circumstances,  when 
the  eyes  are  rotated  there  will  be  a  prismatic  result  of  dif- 
ferent amount  in  each  eye,  and  this  may  mean  diplopia,  or 
at  least  an  exertion  on  the  part  of  the  extraocular  muscles 
to  prevent  diplopia  which  will  cause  dizziness,  nausea,  head- 
ache, etc.  (2)  With  lenses  as  just  mentioned,  the  size  of 
the  two  retinal  images  will  not  be  exactly  the  same,  and  this 
will  mean  an  interference  with  clear  binocular  vision. 


ANISOMETKOPIA.  28 1 

For  purposes  of  study,  the  writer  would  divide  cases  of 
heterometropia  and  anisomctropia  into  four  different  classes. 

CLASS  I. — This  class  embraces  those  cases  in  which  the 
difference  in  the  ametropia  between  the  two  eyes  is  very 
slight  or  does  not  exceed  two  diopters.  In  fact,  there  are 
very  few  pairs  of  eyes  that  are  not  slightly  anisomctropic  ; 
such  eyes  usually  receive  their  exact  corrections  with  com- 
fort, regardless  of  the  condition. 

CLASS  II. — Cases  that  come  under  this  head  also  accept 
their  exact  correction  for  each  eye,  but  do  not  attempt 
binocular  single  vision,  and  may  never  suffer  the  least  in- 
convenience ;  these  cases  are  extremely  rare.  They  do  not 
complain  of  diplopia,  as  they  have  learned  to  ignore  the 
false  image.  Cases  of  alternating  squint,  one  eye  myopic 
and  the  other  eye  hyperopic,  may  be  included  in  this  class. 

CLASS  III. — This  is  a  class  which  will  accept  the  exact 
correction  before  one  eye  only,  and  the  eye  which  has  the 
greatest  amount  of  ametropia  will  refuse  almost  any  lens 
except  the  very  weakest.  The  eye  that  has  the  most  ame- 
tropia is  often  quite  amblyopic. 

CLASS  IV. — This  class  includes  young  children  especi- 
- ,  ally  ;  cases  of  squint.  In  children  the  correction  as  found 
by  the  static  refraction  is  usually  accepted. 

The  Prescribing  of  Glasses  in  Cases  of  Heterometropia 
and  Anisometropia. — Excluding  Class  I,  there  is  no  fixed 
rule  to  follow  when  ordering  glasses  in  decided  cases  of 
heterometropia  or  anisometropia,  and,  in  fact,  such  eyes  are 
a  constant  study  to  the  most  able  ophthalmologist.  The 
younger  the  patient,  however,  the  more  likelihood  of  a  favor- 
able result  from  the  careful  selection  of  a  glass  for  each  eye; 
but  when  the  patient  is  an  adult,  it  becomes  a  very  serious 
question  as  to  what  glass  to  prescribe  that  will  give  satisfac- 
tion. As  good  results  are  to  be  expected  in  children,  they 
24 


282        REFRACTION  AMD  HOW  TO  REFRACT. 

should  receive  the  most  careful  retinoscopic  refraction.  The 
child  comes  under  observation  on  account  of  a  squint,  and  an 
operation  for  the  deformity  is  often  demanded  ;  but  the  opera- 
tion must  be  refused  until  the  ametropia  has  been  carefully 
treated.  Glasses  having  been  prescribed,  the  squinting  eye  is 
put  to  work  to  develop  its  seeing  qualities,  which  have  been 
permitted  to  lie  dormant  for  want  of  a  proper  glass.  To  do 
this,  the  "good"  eye  is  shielded  or  blinded  with  a  hand- 
kerchief tied  over  it,  or  a  blinder  (see  Fig.  1 60)  placed  over 
its  correcting  lens,  for  an  hour  or  two  each  day,  and  in  this 
way  an  attempt  is  made  to  bring  the  vision  in  the  squinting 
eye  up  to  that  of  its  fellow. 

;  Or  another  way  to  develop  the  vision  in  the  squinting 
;eye  is  to  use  a  cycloplegic  in  the  "  good  "  eye,  so  that  the 
squinting  eye  must  do  most  of  the  work.  This  is  rather 
trying  to  the  little  patient,  and  often  means  the  additional 
use  of  dark  glasses.  As  a  rule,  the  "good"  eye  has  the 
least  amount  of  ametropia,  but  occasionally  the  reverse  con- 
dition may  exist. 

In  a  case  like  the  following,  the  little  girl,  five  years  of 
age,  was  brought  on  account  of  convergent  squint  in  O.  S., 
which  developed  or  commenced  to  appear  when  ten  months 
of  age,  and  the  parents  attributed  it  to  the  habit  of  sucking 
her  thumb  at  the  time  of  being  weaned.  Refraction,  with 
atropin  as  the  cycloplegic,  and  obtained  with  the  retino- 
scope,  showed  O.  D.,  -(-2.00  sph.;  O.  S.,  +4.00  sph.  O 
+  1.00  cyl.  axis  75  degrees. 

This  child  developed  the  squint  on  account  of  the 
monocular  astigmatism  and  because  it  could  not  accom- 
modate sufficiently  with  the  left  eye.  To  avoid  diplopia 
at  the  same  time  that  the  eyes  were  converging,  the 
left  eye  naturally  turned  inward.  With  correcting  glasses, 
and  practising  as  above  directed,  the  squint  entirely 


BIFOCALS.  283 


VI 


disappeared,  and  vision    one   year   later    was  -_.-   in   each 


eye  with  the  correcting  glasses.  If  the  glasses  are  laid 
aside  for  any  length  of  time,  the  squint  returns.  This 
child  must  wear  the  glasses  or  have  "  squint." 
/To  make  sure  that  no  injustice  is  done  to  an  apparently 
/amblyopic  eye  in  an  adult  (Class  III,  p.  270)  where  ambly- 
opia  exanopsia  has  existed  for  many  years  and  nothing  has 
been  done  to  improve  its  correction,  the  writer  makes  it  a 
rule  to  prescribe  the  exact  correction  for  each  eye,  and  at  the 
time  of  ordering  the  glasses  explains  to  the  patient  what 
the  purpose  and  desire  is,  and  that  if  there  is  any  great 
amount  of  discomfort  in  any  way,  he  must  return  and  have 
any  necessary  change  made  in  the  glass.  These  patients 
should  be  kept  undef  observation  and  the  amblyopic  eye 
given  some  sort  of  a  correction  and  improved  as  much  as 
possible ;  the  purpose  being  not  to  allow  the  eye  to 
degenerate  or  grow  more  amblyopic,  for  if  any  accident 
should  befall  the  "  good"  eye,  then  the  amblyopic  eye  will 
often  be  a  friend  indeed. 

Glasses  for  Presbyopes  and  Cases  of  Aphakia. — Unless 

I  the  distant  vision  is  improved  or  asthenopic  symptoms  are 

> 
.relieved  by  glasses,  it  will    be  sufficient   to  prescribe  the 

i  near  correction  only.  When  a  distant  and  near  correc- 
tion are  required,  they  may  be  prescribed  as  two  pairs  of 
glasses  in  separate  frames,  or  two  pairs  in  one  frame,  known 
as  bifocals.  Bifocals,  or  what  is  equivalent  to  bifocals,  are 
made  in  different  ways. 

i.  Franklin*  or  Split  Bifocals  (Figs.  175  and  176). — 
This  form  of  bifocals  consists  of  an  upper  and  a  lower  lens, 
each  with  its  individual  center  ;  the  upper  lens  is  for  dis- 
tance and  the  lower  for  near  vision.  Such  lenses  must  have 
the  frame  all'  around  the  edges,  so  as  to  hold  them  in  posi- 

*  "  History  of  Spectacles,"  L.  Webster  Fox,  "  Med.  and  Surg.  Reporter," 
1890,  vol.  LXII,  513-519. 


284 


REFRACTION  AND  HOW  TO  REFRACT. 


tion.  Bifocals  of  this  kind  are  not  in  common  use.  The 
field  of  distant  vision  is  limited  by  the  unnecessarily  large 
near  correction,  and  where  the  two  lenses  come  together, 


A 


FIG.  175. 


there  is  apt  to  develop  chromatic  aberration  and  a  decided 
prismatic  effect  when  the  vision  is  directed  through  this 
space. 

B.    O.  D.,  -|-2.oo  sph. 
O.  S.,  -(-a.oosph. 
SlG. — For  distance. 

B-     O.  D.,  -f  4.00  sph. 
O.  S.,  +4.00  sph. 
SlG. — For  near. 
DIRECTIONS  TO  OPTICIAN. — Make  into  Franklin  or  split  bifocals. 

.  Morck's  Patent  or  "Perfection"  Bifocals  (Fig. 
). — These  are  a  modification  of  the  Franklin  or  split 
bifocals,  and  in  place  of  having 
lenses  united  in  a  horizontal  line, 
the  near  and  distant  lenses  are 
fitted  together  with  correspond- 
ing crescent  edges.  This  form 
of  bifocal  gives  a  larger  field  for 
the  distance  correction,  and,  like 
the  Franklin,  is  much  better 
for  those  who  work  in  a  damp 
atmosphere  and  can  not  wear  the  cement  bifocal.  It  is, 


FIG.  177. 


I 


BIFOCALS. 


285 


however,  more  expensive  than  the  cement  form ;  but,  like 
the  Franklin,  it  often  looks  clumsy  or  heavy  on  account 

of  the  frame.      "Perfection"  or  "Morck"   bifocal  must  be 

j,     .  — "^^Mg*      .  f* 

signified  in  writing  the  prescription.      ^j  ~ftn_&  '  *  '  <  ^  *  f»  • 

3.  Cement  Bifocals-fScTTIgsTi/S  and  179*). — This  is 
the  most  common  form  of  bifocal  and  the  least  expensive 
in  its  original  cost,  as  also  when  making  changes  in  the 
near  correction.  This  bifocal  is  made  by  cementing  a  seg- 

A 


FIG.  181. 


FIG.  182. 


ment  of  a  small  periscopic  sphere  on  to  the  lower  part  of 
the  distance  correction.  This  periscopic  sphere  or  disc  or 
segment,  as  it  is  called,  has  a  prismatic  quality  (see  Fig. 
179)  suitable  to  the  exigencies  of  the  individual  lens  to 
which  it  is  cemented.  The  segment  may  be  of  any  shape 
desired.  Those  in  common  use  are  shown  in  figures  180, 
181,  and  182.  It  is  cemented  to  the  distance  correction 


*  Described  by  Dr.  Geo.  M.  Gould,  "  Med.  and  Surg.  Reporter,"  Nov.  3, 

1888. 


286 


REFRACTION  AND  HOW  TO  REFRACT. 


with  Canada  balsam.      While  this  is  the  usual  method  of 
making  a  cement  bifocal,  yet  it  may  be  made  by  cementing 

IB 


FIG.  183. 


V 

FIG.  184. 


a  concave  segment  to  the  upper  part  of  the  near  correction. 
(Figs.  183  and  184.)     This  form  is  not  in  common  use. 

H.    O.  D.,-|2S. 
O.    S.,  +2  S. 


Cement  on  the  lower  part  of  the  above  O.  D.  and  O.  S.,  -\-2.oo 
SlG.  —  Make  frameless  bifocals. 


Or, 


li.     O.  D.,  +4S. 
O.   S.,  +4  S. 

Cement  on  the  upper  part  of  O.  D.  and  O.  S.,  — 2.00  S. 
SlG.  —  Make  frameless  bifocals. 

4.  Achromatic  Bifocals  (Figs.   185  and   186*). — This 


FIG.  185. 


FIG.  1 86. 


form  of  bifocal  is  used  principally  in  cases  of  aphakia  where 
the  plus  sphere  is  quite  thick  and  correspondingly  heavy.     It 


*  Borsch  patent. 


BIFOCALS. 


237 


is  made  in  one  of  two  ways  :  (i)  By  grinding  out  a  portion 
of  the  lower  part  of  the  distance  correction  (in  crown  glass) 
and  cementing  into  the  concavity  a  biconvex  segment 
of  flint  glass.  This  form  of  bifocal  is  a  combination  of 
the  "  perfection  "  and  lenticular.  (2)  In  place  of  grinding 
the  concavity  in  one  lens,  as  just  described,  this  achromatic 
bifocal  is  also  made  by  taking  two  planoconvex  spheres 
and  grinding  out  a  concavity  in  each,  and  then  inserting  a 
convex  sphere  of  flint  glass,  as  shown  in  figure  186  ;  these 
three  lenses  are  then  cemented  together,  and  when  com- 
pleted, look  like  the  cement  bifocal,  as  shown  in  figure  182. 
It  is  a  matter  for  very  careful  calculation  as  to  just  how 
strong  to  make  the  flint  glass  segment,  so  that  the  result 
may  be  just  exactly  right.  The  merits  of  this  bifocal  are 
lightness  and  the  absence  of  chromatic  aberration.  These 
lenses  are  very  expensive. 

5.  Solid  or  Ground  Bifocals  (Figs.    187  and  188). — 
Lensts  of  this  character  are  made  in  one  piece  by  grinding 


<M^*' 


FIG.  187. 


FIG.  188. 


on  to  the  upper  part  of  the  near  correction  the  necessary  minus 
spheric  correction  for  distance.  They  look  neat,  but  are  npt 
al \vays  comfortable,  on  account  of  the  resulting  prismatic 
effect,  which  is  especially  apt  to  occur  when  the  lens  is  con- 
vex, though  this  may  not  be  so  troublesome  a  feature  when 
the  lens  is  moderately  concave. 


288 


REFRACTION  AND  HOW  TO  REFRACT. 


Single  Crystal  Bifocals. — Lenses  of  this  character  are 
made  in  one  piece,  and  in  this  respect  are  like  the  solid 
bifocal,  but~they  differ  from  the  solid  bifocal  in  two  ways  : 
the  near  correction  is  made  by  grinding  on  to  the  lower 
part  of  the  distance  correction,  the  necessary  plus  sphere. 
The  solid  bifocal  gives  a  decided  prismatic  effect  where  the 


FIG.  189. 


FIG.  190. 


FIG.  191. 


FIG.  192. 


FIG.  193 

two  corrections  meet,  whereas  this  defect  is  said  not  to 
exist  in  the  crystal  bifocals.  These  bifocals  are  expensive. 
6.  Patients  who  have  a  very  weak  distance  correction, 
and  could  do  without  it,  sometimes  accept  it  for  the  con- 
venience of  wearing  bifocals ;  they  do  not  wish  to  be  an- 
noyed by  taking  off  or  putting  on  a  near  correction,  prefer- 


BIFOCALS. 


289 


ring  to  have  the  glasses  where  they  can  find  them ;  business 
men  especially.  Other  patients  prefer  to  do  without  a  dis- 
tance correction,  and  will  often  use  a  near  correction  that 
has  one-third  or  nearly  one-half  of  its  upper  part  cut  away, 
so  that  they  can  look  over  the  near  correction  when  they 
wish  to  see  at  a  distance.  (See  Figs.  189,  190,  191,  192, 
and  193.)  Myopes  who  do  not  need  a  near  correction  will 
wear  their  distance  correction  with  its  lower  portion  cut 
away,  so  that  when  they  wish  to  see  near  at  hand,  tiiey  can 
look  under  the  distance  correction.  £  ^^  Ji^/L^J 

7.  Patients  who   require  a  distance  correction,  and  can 
not  get  accustomed  to   cement  segments,  and"  at  the  same 
time  do  not  wish  to  change  the  distance   correction,  but 
prefer  to  keep  it  on  all  the  time,  can  put  on  their  addition 
for  near  vision  in  the  form  of  hook  or  "grab"  fronts  of  the 
same  size  as  the  distance  lenses  or  reduced  one-half  in  the 
vertical  diameter.      This  is  not  always  a  good  combination, 
as  in  even-  instance  the  lenses  do  not  lie  in  contact  with 
each  other. 

8.  Lorgnettes  may  be  used  as  a  distance  correction  or  as 
a  substitute  for  hook  fronts.     Some  myopic  women  who 
wear    their    near    corrections    constantly   often    carry  lor- 
gnettes, which  they  hold  up  in  front  of  the  near  correction 
to  improve  distant  vision  for  a  few  minutes,  or,  wearing  the 
distance  correction,  can  use  a  plus  lens  in  the  lorgnettes  for 
near  vision. 

9.  Cases  of  monocular  aphakia  where  the  vision  in  the 
fellow-eye  is  very  defective  can  wear  reversible  frames,  one 
lens  for  distance  and  the  other  for  near, — that  is  to  say,  a 
frame  which  has  a  free  joint  at  the  temples, — and  in  this  way 
they  avoid   bifocals,    and  can   change  the  distance  for  the 
near  correction,  by  turning  the  temple-pieces. 

In  some  cases  of  aphakia  where  the  lens  is  very  powerful, 
25 


290 


-  3, 

REFRACTION  AND  HOW  TO  REFRACT. 


a  bifocal  segment  can  sometimes  be  dispensed  with,  if  the 
patient  has  a  long  nose,  by  sliding  the  lens  down  from  the 
eye  and  then  heading  the  reading  matter  at  the  conjugate 
focus.  A  toric  lens  (Fig.  194)  is  very  acceptable  in  occa- 
sional instances,  as  it  reduces  somewhat  the  weight  and 
thickness  of  the  lens,  and  also  enlarges  the  field  of  vision. 
A  toric  (torcine  or  toriqiic,  "twisted")  lens  is  one  which 


Copyright,  1886,  by  Chas.  F.  Prentice. 
FlG.   194. 

has,  combined  in  one  surface,  the  optic  effects  of  a  sphero- 
cylindric  lens,  or  two  cylinders  of  different  strength  at  right 
angles  tp  each  other.  Unfortunately,  this  form  of  a  lens  is 
quite^xpensive. 

General  Considerations. — Before  prescribing  any  pair 
of  glasses,  the  patient  should  have  the  opportunity  to  wear 
the  correction  in  the  office  for  a  short  time,  that  he  may 
study  its  effect;  this  is  especially  necessary  (i)  when  the 
glasses  are  strong  ones ;  (2)  when  there  is  monocular  as- 
tigmatism ;  (3)  when  one  lens  is  much  stronger  than  the 
other  (anisometropia)  ;  (4)  when  the  astigmatism  is  asym- 
metric ;  or  (5)  when  there  is  a  strabismus,  etc.  The  patient 
loses  confidence  (and  the  surgeon  is  not  made  happy)  when 
the  patient  returns  with  his  glasses  in  his  hands  and  states 
that  he  can  not  wear  them — that  they  make  him  "  dizzy  "  or 
"  tipsy  "  ;  that  the  glasses  make  the  pavement,  houses,  trees, 


BIFOCALS.  291 

people,  pictures  on  the  wall,  chairs,  tables,  etc.,  all  appear 
as  if  they  were  going  to  fall  to  one  side.  The  surgeon 
should  have  anticipated  all  this,  and  assured  the  patient 
beforehand  that,  after  a  little  perseverance  and  practice,  this 
distortion  (parallax)  will  disappear ;  and  if  not,  then  a 
change  will  have  to  be  made  in  the  glasses.  Very  often 
the  whole  difficulty  is  due  to  a  want  of  proper  centering  of 
the  lenses,  presuming,  of  course,  that  the  glasses  ordered 
are  perfectly  correct. 

Patients  who  require  weak  lenses — spherocylinders  or 
cylinders  alone — may  at  some  time  be  informed  that  "  the 
correction  is  but  window-glass,"  and  thus  the  surgeon  may 
be  put  in  disgrace  as  having  prescribed  for  mercenary 
reasons,  when  in  truth  the  glasses  have  already  cured  an 
old  blepharitis  or  asthenopia.  In  ordering  weak  correc- 
tions, therefore,  the  character  and  purpose  of  the  glasses! 
should  be  imparted  to  the  patient. 

It  is  interesting  to  notice  that  strong  glasses  are  usually 
ordered  to  improve  the  vision,  and  not  always  for  the  relief 
of  asthenopia,  whereas  weak  corrections  are  prescribed  for 
the  relief  of  headaches,  etc.,  without  any  decided  improve- 
ment in  the  vision  which  the  patient  can  appreciate  when 
looking  at  a.  distance,  and  many  such  patients  will  say  they 
can  see  just  as  well  without  their  glasses.  When  strong 
plus  spheres  are  prescribed  for  a  child,  it  will  do  no  harm 
to  inform  the  parents  of  the  character  of  the  glasses,  so 
that  when  a  presbyope  tries  the  child's  glasses,  the  sur- 
geon may  not  be  accused  of  ruining  the  child's  eyes  by 
having  ordered  a  pair  of  glasses  strong  enough  for  a  grand- 
mother to  read  with,  and  the  child  hurried  off  to  a  rival 
confrere  to  have  the  "  outrage  "  rectified. 

A  patient  who  has  fought  against  the  inevitable,  using 
headache  powders,  liver  pills,  etc.,  in  the  vain  hope  of  not 


2Q2        REFRACTION  AND  HOW  TO  RKFRACT. 

having  to  put  on  glasses,  may  still  object  to  their  use  for 
various  reasons.  It  may  be  that  glasses  will  not  add  to 
the  personal  appearance,  or  the  parents  may  dislike  the 
idea,  fearing  that  "  the  oculist  puts  glasses  on  every 
patient,"  or  that  "  the  eyes  will  never  be  the  same  again," 
or  that  "  the  habit  of  wearing  glasses,  once  established,  can 
never  be  stopped."  These  and  many  other  statements  will 
serve  to  enliven  the  daily  routine  of  ophthalmic  practice. 
These  objections  having  been  met  from  the  point  of  view 
of  the  patient's  individual  welfare  and  future  good  of  his 
eyes,  the  next  question  that  arises  is  what  form  of  glasses 
shall  be  prescribed. 

Spectacles. — The  child  is  certainly  a  candidate  for  spec- 
tacles. The  frames  must  be  very  durable,  and  preferably 
of  14-carat  gold.  Spectacle  frames  keep  the  lenses  in  posi- 
tion, and  the  lenses  are  then  less  liable  to  be  broken  than 
in  the  form  of  eye-glasses,  and  for  most  occupations  are  to 
be  preferred.  Occasionally,  the  shape  of  the  nose  will  pre- 
clude the  use  of  anything  else  but  spectacles.  When  one 
lens  is  very  heavy  or  both  have  considerable  weight,  or 
when  one  or  both  lenses  are  cylindric,  with  axes  inclined, 
spectacles  are  certainly  indicated. 

Eye-glasses,  also  called  "  pinc-nez,"  are  for  the  adult, 
and  may  be  prescribed  when  the  lenses  are  not  too  heavy, 
or  the  cylinders  too  strong  or  their  axes  inclined.  Kve- 
glasses  are  easily  bent,  and  lose  their  exact  positions  before 
the  eyes.  For  the  young  society  girl  nothing  but  the  most 
delicately  made  eye-glasses  will,  as  a  rule,  be  accepted. 

Bifocals. — These  should  not,  as  a  rule,  be  prescribed  if 
the  lenses  are  very  strong  or  the  correction  a  complicated 
one,  or  the  patient  advanced  in  years  and  has  never  at- 
tempted them  before,  or  if  the  patient  is  very  portly  or 
uncertain  in  his  gait,  or  the  vision  is  not  brought  close  to 


BIFOCALS.  293 

the  normal.  Two  separate  pairs  of  glasses  are  to  be  recom- 
mended under  these  circumstances.  When  ordering  any 
pair  of  bifocals,  the  patient  should  be  cautioned  and  in- 
structed that  when  looking  downward,  going  up  or  down 
stairs,  getting  into  or  out  of  a  conveyance,  he  is  to  look  to 
one  side  or  over  the  segment  of  the  bifocal  and  not 
through  it,  otherwise  he  will  be  liable  to  make  a  false  step 
or  misjudge  the  distance,  which  might  mean  serious  bodily 
injury,  for  which  the  surgeon  does  not  wish  to  hold  himself 
responsible. 

Glasses  for  constant  use  should  be  placed  perpendicularly 
or  at  an  axis  of  about  5  degrees  to  the  plane  of  the  face, 
with  the  optic  centers  corresponding  to  the  pupillary  cen- 
ters when  the  eyes  are  directed  to  a  distance.  If  the  lenses 
are  unusually  strong  and  to  be  used  principally  at  near- 
work,  then  it  may  be  necessary  to  consider  the  advisability 
of  having  two  pairs  of  glasses,  one  for  distance  and  one 
for  near,  each  with  the  centers  to  answer  for  the  object  in 
view.  If  only  one  pair  of  glasses  has  been  ordered,  and 
they  happen  to  be  very  strong,  then  a  pair  of  prisms  in 
hook  fronts  may  have  to  be  used  at  the  near-work,  so  as  to 
counteract  the  prismatic  effect  of  looking  through  the  dis- 
tance glasses  during  convergence.  Glasses  for  near-work 
only  should  be  put  into  a  frame  made  especially  for  the 
purpose,  so  that  the  lenses  may  have  an  inclination  in 
keeping  with  the  downward  turn  of  the  eyes,  and  thus  be 
perpendicular  to  the  axis  of  the  eyes,  and  the  lenses  should 
be  decenterecl  inward  to  equal  the  convergence.  The  one 
serious  objection  to  bifocals  in  certain  instances  is  that  the 
glasses  can  not  be  made  with  the  inclination  suitable  for 
both  distance  and  near  vision,  and  very  often  there  must  be 
a  compromise  between  the  two. 

The  surgeon  should  make  it  a  point  to  carefully  inspect 


294         REFRACTION  AND  HOW  TO  REFRACT. 

every  pair  of  glasses  which  he  orders,  as  his  painstaking 
efforts  and  best  endeavors  may  be  completely  frustrated  by 
poorly  fitting  lenses. 

1.  The  lenses  should  neutralize.     (See  p.  58.) 

2.  The  optic  centers  should  be  at  the  points  indicated. 

3.  The  cylinder  axes  must  be  exact. 

4.  The  lenses  must  be  perpendicular  or  inclined  to  the 
front  of  the  eye,  as  necessary. 

5.  The  distance  of  the    lenses    from    the    eyes    should 
always  be  sufficient  to  clear  the  lashes ;   and  if  these  are 
very  long,  they  may  have  to  be  trimmed. 

6.  The  most  convex  or  the  least  concave  surface  of  the 
lens  should  be   placed  away  from  the  eyes.     Or  the  most 
concave  surface  toward  the  eye. 

7.  The  lenses  should  be  of  the  correct  size  for  the  indi- 
vidual face.     These  and  many  other  points  for  the  average 
case  must  receive  the  careful  consideration  of  the  surgeon. 

Tinted  or  Colored  Glasses. — Except  for  the  relief  of 
photophobia  following  cataract  extraction,  mydriasis,  or 
inflammatory  diseases,  the  surgeon  does  not  order  colored 
glasses.  Colored  lenses  are  to  be  deprecated  except  in  the 
cases  just  mentioned,  as  they  only  increase  the  tendency  to 

jiotophobia  instead  of  correcting  it. 

[metric  Lenses. — These  are  made  to  conform  in  out- 
line to  the  normal  field  of  vision  as  recorded  by  the  per- 
imeter, hence  the  name.* 

The  usefulness  of  the  perimetric  lens  is  limited  to  those 
cases  in  which  the  correction  contains  a  plus  cylinder  and 
the  lens  is  of  moderate  strength.  It  is  not  a  lens  that  can 


*  The  writer  described  this  form  of  lens  before  the  Section  in  Ophthalmology 
of  the  College  of  Physicians  of  Philadelphia,  in  March,  1897. 


TRIFOCALS. 


295 


be  prescribed  in  myopia  or  aphakia.  The  purpose  of  the 
perimetric  lens  is  to  give  a  normal  field  and  have  the  edge 
of  the  lens  sufficiently  removed  that  the  patient  may  not  be 
disturbed  by  seeing  it.  It  certainly  enlarges  the  field  of 
vision,  and  in  this  way  is  a  great  advantage  in  certain  occu- 
pations, playing  the  piano,  etc.  Figure  203  or  204  may 
answer  the  same  purpose  if  properly  centered. 

Trifocals. — Occasionally,  a  patient  is  not  content  with 
bifocals,  but  will  demand  a  focal  point  somewhere  between 
infinity  and  his  working  distance;  this  can  only  be  produced 
by  cementing  two  segments  of  different  sizes  and  strength 


Fie.  195. 


FIG.  196. 


on  the  intermediate  correction.  (Figs.  195  and  196.)  Book- 
keepers who  have  to  work  at  large  and  lengthy  ledgers  find 
great  comfort  in  this  combination,  though  to  be  of  special 
service  the  lenses  must  be  made  large.  Example:  +2.00 
equals  working  distance  at  I  meter.  Minus  I  diopter 
added  above  equals  infinity  vision.  -+-2.00  added  below 
gives  near  vision  at  1 3  inches. 

Decentering  of  Lenses. — Instead  of  writing  a  prescrip- 
tion for  a  lens  and  prism,  the  prismatic  effect  of  the  lens 
may  be  obtained  by  decentering  the  lens.  The  rule  is 


296        REFRACTION  AND  HOW  TO  REFRACT. 

that  for  every  centimeter  of  decentering  there  will  result 
just  as  many  prism-diopters  as  there  are  diopters  in  the 
meridian  of  the  correcting  lens.  For  example,  +4  sph-  O 
4  P.  D.,  base  out,  is  the  same  as  +4  sph.  decentered  I  cm. 
outward  ;  or  -(-4  sph.  O  2  P.  D.,  base  in,  equals  -f  4  S. 
decentered  5  mm.  inward  ;  or  -f-2  sph.  O  +2  cyl.  axis 
90  degrees  O  2  A,  base  outward,  equals  -\-2  sph.  O  -f  2 
cyl.  axis  90,  decentered  5  mm.  outward. 

While  it  is  well  for  the  student  to  know  how  to  decenter 
lenses,  yet  the  writer  does  not  recommend  such  lenses, 
preferring,  when  necessary,  to  order  a  prismatic  combina- 
tion, and  have  the  optician  fill  the  prescription,  starting 
direct  from  the  prism. 


CHAPTER  XII. 

LENSES,  SPECTACLES,  AND  EYE-GLASS 
FRAMES.  HOW  TO  TAKE  MEASURE- 
MENTS FOR  THEM  AND  HOW  THEY 
SHOULD  BE  FITTED. 

The  selection  of  the  size  and  shape  of  lenses,  the  char- 
acter of  the  spectacle  and  eye-glass  frames  and  their  adjust- 
ment, is  the  work  of  the  optician.  It  occasionally  happens, 
however,  that  the  surgeon  may  not  have  an  optician  in  his 
town,  and  will,  therefore,  have  to  take  the  necessary  meas- 
urements himself  and  send  them,  with  his  prescription,  to 
an  optician  in  a  neighboring  city.  This  chapter  is  there- 
fore added  for  the  benefit  of  such  surgeons.  It  is  hardly 
necessary  to  state  that  the  frames  should  be  very  carefully 
adjusted  and  the  lenses  centered  to  the  patient's  eyes.  A 
lens  improperly  adjusted  may  utterly  destroy  the  good 
effect  of  the  most  skilfully  selected  correction,  giving  dis- 
comfort to  the  patient  and  reflecting  seriously  upon  the 
surgeon's  ability.  In  fact,  it  is  always  well  for  the  surgeon 
to  personally  inspect  every  pair  of  glasses  which  he  may 
order. 

Lenses. — These  are  spoken  of  as  "eyes,"  and  come  in 
various  sizes  and  shapes.  They  are  spoken  of  as  O,  double 
O  (OO),  triple  O  (OOO),  etc.  (See  Figs.  202,  203,  204, 
205,  and  206.)  Or  sizes  smaller  than  O  are  numbered  I,  2, 
3,  or  4.  (See  Figs.  197,  198,  199,  200.)  Different  shapes 
and  sizes  are  lettered  A,  B,  C,  D,  F,  or  X.  (See  Figs.  189, 
190,  191,  192,  193,  201.)  All  these  lenses  are  also  marked 

297 


298 


REFRACTION  AND  HOW  TO  REFRACT. 


in  millimeters  of  breadth  and  length.  The  lenses  for  indi- 
vidual patients  are  selected  according  to  the  purpose  for 
which  they  are  intended,  and  particularly  to  be  in  keeping 


FIG.  197. 


FIG.  199. 


FIG.  201. 


FIG.  198. 


FIG.  200. 


FIG.  202. 


with  the  facial  measurements.  The  size  or  "  eye  "  O  (39  X  30 
mm.)  is  the  usual  size  for  the  average  adult,  and  number 
2,  3,  or  4  is  for  a  child.  C,  D,  or  F  may  be  ordered  for  a 


LENSES. 


299 


presbyope  who  does   not  need  a  distance  glass  and  who 
does  not  wish  to  be  taking  off  the  near  correction  to  see  at 


FIG.  205. 


FIG.  206. 


a  distance  ;  in  other  words,  such  a  shaped  lens    can  be 
looked  over  without  any  difficulty.      Or  the  presbyope  who 


3OO        REFRACTION  AND  HOW  TO  REFRACT. 

requires  a  — 2  for  distance  and  can  see  to  read  without 
any  near  correction, — being  about  fifty  years  of  age, — could 
have  his  minus  lenses  made  in  the  shape  of  A,  B,  C,  or  D 
inverted,  and,  wearing  this  for  distance,  would  look  under 
it  when  he  wished  to  see  near  at  hand.  As  a  rule,  the 
patient  with  a  narrow  face  and  short  interpupillary  distance 
will  require  a  small  "eye,"  whereas  the  patient  with  a 
broad  face  and  long  interpupillary  distance  will  require  a 
large  "eye." 

Spectacle  Frames  (Fig.  211). — These  consist  of  a  nose- 
piece  (called  the  bridge)  and  temples  (called  sides).  These 
are  attached  to  the  lenses  ("eyes")  by  screws  passing 
through  holes  which  have  been  drilled  through  them,  mak- 
ing what  is  known  as  the  frameless  spectacles  ;  or  a  wire  is 
fitted  around  the  lenses,  to  which  the  bridge  and  sides  are 
attached  with  solder,  forming  the  "framed"  spectacles. 

Eye-glass  Frames  (Fig.  212). — These  consist  of  a 
spring  and  nose-pieces  ;  the  latter  are  called  guards. 
Framed  and  frameless  eye-glasses  have  the  nose-pieces  or 
guards  attached  to  the  lenses  as  in  the  spectacles. 

How  to  Take  Measurements. — There  are  three  points 
that  require  particular  attention  :  (i)  The  center  of  the 
lens  should  correspond  with  the  center  of  the  pupil ;  (2) 
the  lens  must  be  just  far  enough  from  the  eyes  to  avoid  the 
lashes,  and  if  these  are  very  long,  they  must  be  trimmed  ; 
(3)  the  lens  must  be  at  such  an  angle  that  the  visual  axis 
will  be  perpendicular  to  it. 

First  Measurement. — The  Interpupillary  Distance. — To 
accurately  measure  the  distance  from  the  center  of  one 
pupil  to  the  center  of  the  other  is  not  always  an  easy  thing 
to  do,  especially  if  the  pupils  are  dilated  ;  hence,  it  is  good 
practice  to  measure  this  distance  from  the  inner  side  or  edge 
of  one  pupil  to  the  outer  edge  of  the  other.  This  measure- 


FIRST    MEASUREMENT. 


301 


ment  can  be  made  with  an  ordinary  rule  divided  to  six- 
teenths of  an  inch  or  in  millimeters,  or  with  a  special  instru- 
ment for  the  purpose,  called  a  pupilometer.  The  patient 
is  told  to  look  directly  to  the  front,  at  an  object  across  the 
room,  and  the  surgeon,  in  front,  with  his  head  nearly  in  the 
line  of  sight,  holds  the  rule  across  the  patient's  face,  as 
close  as  the  bridge  of  the  nose  or  eyelashes  will  permit. 
With  his  thumb-nail  as  a  marker,  the  surgeon  gages  the 
distance  as  indicated  (see  Fig.  207),  which  illustrates  the 
conditions.  In  taking  this  measurement  the  surgeon  should 
be  at  an  arm's  length  from  the  eyes,  for  the  reason  that  his 


FIG.  207. 

own  eye  forms  the  apex  of  a  triangle  of  which  the  eyes  of 
the  patient  form  the  base,  and  the  measurement  is  apt  to 
be  two  or  three  or  four  millimeters  short  if  he  gets  too 
close. 

If  the  glasses  are  to  be  worn  for  distance  only,  then  the 
measurement  must  be  for  the  full  interpupillary  distance,  as 
the  patient  looks  into  infinity  ;  but  if  the  glasses  are  for  near- 
work  only,  then  the  distance  between  the  pupils  must  be 
correspondingly  diminished,  and  the  measurement  taken 
as  the  patient  looks  at  a  near  point.  If  the  glasses  are 
to  be  worn  for  both  near  and  far  vision,  for  constant  use, 


3O2        REFRACTION  AND  HOW  TO  REFRACT. 

then  the  center  of  the  lenses  must  be  placed  intermediate 
between  the  distance  and  near  measurements. 

Second  Measurement. — The  Bridge. — The  regulation 
spectacle  bridge  is  known  as  the  saddle-bridge,  and  should 
conform  to  the  exact  shape  of  the  patient's  nose.  It  is  in- 
tended to  remain  in  just  one  place,  and  that  is  at  the  bridge 
of  the  nose  (see  B  in  Figs.  208  and  209),  the  place  where 
the  nose  begins  to  extend  outward  after  passing  down  from 
the  forehead.  The  points  B  and  D,  as  shown  in  figure  210, 
represent  the  widest  part  or  base  of  the  bridge.  A  and  R 
are  the  arms,  which  extend  upward  or  outward  and  are 
fastened  to  the  lenses.  The  length  of  the  arms  controls  in 


FIG.  208.  FIG.  209. 


great  part  the  distance  of  the  lenses  from  the  eyes.  To 
raise  or  lower  the  position  of  the  lenses  in  front  of  the  eyes, 
the  posts  or  arms  alone  should  be  bent ;  the  bridge  itself 
should  never  be  tilted,  as  its  edge  will  cut  into  the  skin  of 
the  nose  ;  this  is  a  most  important  consideration  for  the 
patient's  comfort. 

The  Shape  and  Size  of  the  Bridge. — To  take  this  meas- 
urement, the  surgeon  should  have  a  piece  of  lead-wire  or 
thin,  pliable  copper-wire  ;  the  lead-wire  is  best.  This  wire 
is  accurately  molded  to  the  bridge  of  the  patient's  nose, 
the  arms  (A  and  R)  are  bent  to  the  proper  angle,  and  then 
the  ends  of  the  wire  are  curved  or  bent  outward  to  show 
the  plane  of  the  lenses.  (See  Fig.  210.) 


THIRD    MEASUKKMKNT FOURTH     MKAMKKM  KNT.         303 

When  the  wire  has  been  'Dent  into  plaee  and  the  eyelashes 
do  not  touch  at  L  and  L,  it  is  removed  and  placed  on  the 
under  surface  of  a  piece  of  paper,  when  an  impression  and 
lead-pencil  tracing  is  made  of  it.  If  the  measurement  is 
not  taken  in  this  way,  then  the  surgeon,  with  a  pair  of 
moderately  blunt-pointed  compasses,  measures  the  breadth 
of  the  nose  from  B  to  D,  and  also  the  height  of  the  bridge 
from  F  to  E.  The  height  of  the  bridge  is  spoken  of  as 
"out  "  or  "  in  "  ;  the  former  when  F  extends  beyond  the 
plane,  and  "in"  when  F  is  behind  the  plane  of  the  lenses. 
(See  Figs.  208  and  209.) 

Another  good  way  to  take  the  foregoing  measurements 
is  to  have  several  ordinary  steel  frames  of  different  sizes  and 


FIG.  210. 

shapes,  using  whichever  one  of  these  seems  to  fit  the  best, 
and  then  making  any  additional  alterations  in  the  measure- 
ments that  may  be  required. 

Third  Measurement. — This  is  the  length  of  the  sides  or 
temples.  This  measurement  is  taken  from  the  top  of  the 
ear  to  the  plane  of  the  lens,  or  a  horizontal  line  extending 
out  from  the  eyelashes. 

Fourth  Measurement. — The  Size  of  the  Lenses. — This 
will  depend  upon  the  breadth  of  the  face,  the  amount  of 
space  taken  up  by  the  bridge,  its  arms  and  attachments,  as 
also  the  space  occupied  by  the  hinge  and  attachment  of  the 
temples.  Ordinarily,  as  stated  before,  the  adult  will  select 
size  O  and  the  child  No.  2. 


304 


REFRACTION  AND  HOW  TO  REFRACT. 


The  following  blank  is  a  good  guide,  as  covering  all 
the  necessary  measurements  as  referred  to  in  this  descrip- 
tion for  ordinary  glasses. 

STYLE  OF   BLANK   FOR   THE   SURGEON   TO   FOLLOW  WHEN 
ORDERING  GLASSES  FOR  HIS  PATIENT. 

Patienfs  Name, 

Forward  to,      


FIG.  211. 


FIG.  212. 


R. 


O.  D. 
O.  S. 


Distance  or  Near  Frames. 


Frames  of . 


MEASUREMENTS. 
Spectacles.  Eye-glasses. 

Interpupillary  distance, Interpupillary  distance,      .    .    . 

Height  of  bridge, Length  of  guard,  W  to  T,     .    . 

Base  of  bridge, Width  at  base,  W  to  D,     .    .    . 

Shape  of  bridge  (see  drawing),     .    .         Width  at  top,  T  to  P,     .    .    .    . 

Bridge,  "  in"  or  "out," Length  of  arm  of  guards,      .    . 

Length  of  temples, Shape  of  spring  (see  drawing), 

Size  of  "eye," Size  of  "eye," 

Additional  notes, 


Date, 


,M.D. 


STYLE    OF    FRAMES. 


305 


Style  of  Frames.— If  the  glasses  are  to  be  worn  con- 
stantly, they  should  be  perpendicular  or  inclined  about  5 
degrees  from  the  perpendicular  to  the  front  of  the  eyes. 
(See  Fig.  213.)  They  are  spoken  of  as  "  distance  "  frames. 


FIG.  213. 


FIG.  214. 

If  the  glasses  are  to  be  worn  only  at  near-work,  then  the 
lenses  should  be  tilted  downward  ;  this  is  known  as  the 
"near"  frame.  (See  Fig.  214.) 

Fitting   Eye-glasses. — The  position  of  the  lenses  ap- 
plies equally  well  to  eye-glasses.     The 
principal  measurement  is  for  the  nose- 
pieces  or  guards  and   the  arms  or  off- 
sets from  the  guards.      (See  Fig.  215.) 
The  width  of  the  patient's  nose  where 
\Y  and  I),  and  also  T  and  P,  will  press, 
depends,  of  course,  upon  the  length  of 
the  guard  itself — usually  about  14  mm. 
It  is  also  necessary  to  measure  the  posi- 
tion of  the  guards  relative  to  the  plane 
of  the  lenses  ;  that  is,  whether  the  arms  should  be  lon^, 
medium,  or  short,  and  whether  they  are   "out"   or   "in" 
26 


306        REFRACTION  AND  HOW  TO  REFRACT. 

from  the  plane  of  the  lenses.  The  style  of  spring  is  usu- 
ally that  shown  in  figure  215. 

Bifocals. — The  measurements  for  bifocals  are  the  same 
as  for  the  spectacle  or  eye-glass,  except  the  size  and  shape 
of  the  segment,  and  this  should  never  extend  above  the 
median  line  of  the  lens,  and  seldom  to  it. 

Quality  of  Frame. — These  are  made  of  silver,  steel, 
aluminium,  or  gold  ;  the  latter  are  always  to  be  preferred, 
as  more  durable  in  every  way.  Silver  and  aluminium  bend 
easily,  and  steel  frames  rust  and  break.  Every  surgeon 
who  does  his  own  fitting  should  possess  a  small  screw- 
driver, two  pairs  of  delicate  and  yet  strong  pliers  (one  with 
round  and  the  other  with  flat  ends),  and  also  a  small  rule. 


INDEX. 


A. 

ABDUCTION,  180 
Aberration,  negative,  175 

positive,  173 
Absorption  of  light,  1 2 
Accommodation,  65,  66 

amplitude  of,  69,  70 

at  different  ages,  70 

at  rest,  68,  69,  245 

binocular,  84 

cramp  of,  266,  267 

diminution  of,  70 

in  emmetropia,  70,  71 

in  hyperopia,  71,  72 

in  myopia,  72,  73 

in  presbyopia,  70 

mechanism  of,  66,  67 

muscle  of,  66,  67 

observer's,  06,  Q7 

paralysis  of,  216,  217,  218 

patient's,  96,   158 

range  of,  69,  70 

relaxed,  70,  97 
proof  of,  245 

spasm  of,  266,  267 
Acuteness  in  astigmatism,  78 

in  emmetropia,  103 

in  hyperopia,  106 

in  myopia,  115 

of  vision,  63,  64,  78,  79 

record  of,  78,  79 
Adduction,  180,  181 
Aerial  image,  100 
Age,  70,  224,  228 
Albino,  93 

Alternating  strabismus,  196 
Amblyopia,  157 
Amblyoscope,  203,  204 
Ametrometer,  148,  149 
Amrtropia,  105 

axial,  105,  119,  1 20,  121 

curvature,  105,  122 


Angle  alpha,  87 

critical,  20 

gamma,  85 

limiting,  20,  62,  63 

meter,  83,  84 

of  convergence,  83 

of  deviation,  24 

of  five  minutes,  64 

of  incidence,  22 

of  refraction,  21,  22,  23 

of  strabismus,  199 

of  view,  6 1 
Anisometropia,  280,  281 

classification  of,  280,  281 

correction  of,  281,  282 
Anterior  focal  point,  60 

focus,  60 

Apex  of  prism,  23 
Aphakia,  157,  278 

causes  of,  278 

diagnosis  of,  278,  279 

treatment  of,  279 
Aqueous  humor,  60 
Asthenopia,  219,  220 

accommodative,  221,  222 

muscular,  183,  184,  221 

retinal,  220,  221 

treatment  of,  220,  221,  222,  223 
Astigmatic  charts,  137 

clock-dial,  138,  139 

lens,  123 
Astigmatism.     (See  Chapter  V.) 

against  the  rule,  131 

asymmetric,  130 

causes  of,  124 

compound  hyperopic,  127,  128, 

2S6»  25 7.258 

myopic,  128,  259,  260,  261 
corneal,  123 
diagnosis  of,  133 
estimation  of,  124.    (See  Chap- 
ter VI.) 


307 


308 


INDEX. 


Astigmatism,  helerologous,  132 

heteronymous,  132 

homologous,  132 

homonymous,  132 

irregular,  124,  265,  266 

lenticular,  124 

mixed,  128,  129,  261,  262,  263, 
264 

physiologic,  124,  125 

principal  meridians  of,  126 

regular,  125 

shape  of  disc  in,  153 

simple  hyperopic,  126,  250,  251, 

252,  253 
myopic,  127,  254,  255,  256 

statistics  of,  243 

symmetric,  129,  130 

symptoms  of,  133 

tests  for,  133 

treatment  of,  250,  251,  252,  253 

with  the  rule,  131 
Astigmia,  122 
Astigmic,  122 
Atropin,  213 
Axiom,  157 
Axis  of  astigmatism,  126 

of  cylinder,  44 

optic,  60,  85 

principal,  15,  32 

secondary,  15,  36 

visual,  85,  86 
Axonometer,  170 

B. 

BAND  of  light,  169. 
Base  of  prism,  23 
Beam  of  light,  1 1 
Biconcave  lens,  31,  55 
Biconvex  lens,  30,  54 
Bifocals,  283,  284,  285,  286,  287 
Binocular  accommodation,  84 

fixation,  84 
Blepharitis,  no 
Borsch,  286 
Brachymetropia,  113 
Briicke,  muscle  of,  66 
Burnett,  122 

c. 

CAMERA,  65 
Capsule,  68 
Cardinal  points,  59,  60 


Cards,  74,  75,  76,  77 
Cataract,  276 
Catoptrics,  9 
Center  of  fixation,  86 

of  rotation,  86 
Centering  of  lenses,  294 
Chalazion,  124 
Choroid,  94,  95 
Chromo-aberration    test,    145,    146, 

147,  148 
Ciliary  body,  66 
muscle,  66 

anatomy  of,  66 
Cobalt-blue    glass,    113,    145,    146, 

147,  148 
Cocain,  91,  215 
Compound  system,  60 
Concave  lenses,  31,  37 
mirror,  15,  16,  17 

in  retinoscopy,  159 
Concomitant  squint,  196 
Condensing  lens,  99 
Confusion  letters,  134 
Conic  cornea,  172 
Conjugate  foci,  34 
Conjunctiva,  no 
Convergence,  83,  84 
amplitude  of,  85 
angle  of,  84 

insufficiency  of,  183,  184 
negative,  87 
positive,  86 
range  of,  84,  85,  86 
Convergent  strabismus,  195 
Convex  lenses,  30 
Coquilles,  212 
Corneal  reflex  test,  133 
Cover  chimney,  159 

test,  184 
Cramp  of  accommodation,  218,  266, 

267 

Cretes'  prism.  189 
Crossed  diplopia,  179 
Crystalline  lens,  68 
Cycloplegia,  216,  217,  218 
Cycloplegics,  158,  208,  209,  210,  211, 

212,  213,  215,  216 
Cylinder  lenses,  43 

action  of,  43,  44 
axis  of,  43,  230 
combination  of,  49 
crossed,  52,  231 
neutralization  of,  56,  57 


INDEX. 


309 


D. 

DARK  glasses,  212 

room,  91 
Daturin,  213 

Decentering  lenses,  295,  296 
Deorsumduction,  181 
DeSchweinitz,  191 
De/.eng,  102,  175 
Deviation,  angle  of,  24 

estimating  size  of,  199,  200,  201 

in  strabismus,  199 
Diopter,  41,42 
Dioptrics,  9 
Dioptric  system,  60 
Diplopia,  29,  178,  179 

correction  of,  29 

Direct  method,  90,  91,  153,  154,  155 
Disc,  optic,  93 

perforated,  141,  142 

pin-hole,  47,  266 

Placido's,  134,  135 

shape  of  optic,  93 
Distant  type,  81,  82 
Divergence,  83,  196,  197 
Divergent  strabismus,  196 
Duboisin,  213 
Dynamic  refraction,  234,  235 


E. 

ELASTICITY  of  lens,  68 
Electric  light,  102,  175,  220 

blindness  from,  220 
Elongation  of  eyeball,  242 
Epilepsy,  222 
Emergent  rays,  10 
Emmetropia,   103 

description  of,  103,  104,  105 
Erect  image,  94,  95,  96 
Esophoria,  184 

diagnosis  of,  185 

treatment  of,  189,  193,  194 
Ksotropia,   iS,  n;5 
Exophoria,  184,  268,  269 

diagnosis  of,  185 

treatment  of,  189 
Exotropia,  196 
Eye  (frontispiece),  59,  60 
Eye  drops,  208 

emmetropic,  60 

glasses,  292 

hyperopic,  71 


Eye,  myopic,  113 
schematic,  156 
section  of  (frontispiece), 
standard,  59,  60 
-strain,  219,  220,  221,  222,  223 


F. 

FACE,  asymmetric,  130 

broad,  300 

narrow,  300 
Facial  illumination,  162 
Far  point,  68,  69 
Finger  exercise,  191 
Fitting  of  spectacles.     (See   Chan- 
ter XII.) 
Focal  interval,  123 

length,  33 

points,  59,  60 
Focus,  12,  15 

anterior,  33 

conjugate,  34 

negative,  12,  35 

ordinary,  35 

positive,  12,  15 

posterior,  33 

principal,  15,  33 

real,  12,  33 

virtual,  12,  35 

Fogging  method^  232,  233,  234 
Form  of  illumination,  165 
Formation  of  images,  38,  39,  40 
Fox,  283 
Fusion  tubes,  203,  204 


G. 

GLASS,  crown,  22 

flint,  22 

Glasses.     (See  Lenses.) 
Glaucoma,  211 
Gould,  77,  191 
Green,  137 


H. 

HKI.MIIOLTZ,  103 
Heredity,  108,  1 16 
Heterometropia,  279,  280 
Hcteronymous  images,  179 
Heterophoria,  182 


3io 


INDEX. 


History,  224,  225 

Homatropin,  91,  213,  214,  215,  216 
Homonymous  images,  178 
How  to  refract.     (See  Chapter  IX.) 
Hyoscyamin,    213 
Hyperesophoria,  182 
Hyperexophoria,   182 
Hypermetropia,  36,  106 
Hyperopia,   36,    71,    106,    244,    245, 
246,  247 

absolute,  109 

acquired,  278 

amount  of,  72,  121 

axial,  105 

causes  of,  108 

description  of,  106,  107,  108 

diagnosis  of,  112,  113 

estimation  of,  247 

facultative,  108 

latent,  109 

length  of  eyeball  in,  106,  247 

manifest,  109 

relative,  109 

symptoms  of,  no 

total,  109 

treatment  of,  237,  245 
Hyperopic    astigmatism,    126,    127, 

128 

Hyper  phoria,  179,  186,  194 
Hypertropia,  182,  196 


I. 

ILLITERATE  card,  75,  76 

Illiterates,  75,  76 

Illuminated    area.      (See    Figs.   86, 

87,  88.) 
Illumination,  162 

facial,  162 

retinal,  162 
Images,  177 

catroptric,  9 

crossed,  179 

formation  of,  38,  39,  40 

formed  by  mirrors,  14,  15,  16 

heteronymous,  179 

homonymous,  178 

in  astigmatism,  126  to  129 

in  emmetropia,  65,  100,  101 

in  hyperopia,  65 

in  myopia,  65 

in  retinoscopy,  164,  165 


Images,  inverted,  17,  99,  TOO 

on  cornea,  162 

on  lens,  162 

real,  16,  38 

retinal,  61,  98 

size  of,  61,  62,  64 

virtual,  16,  18,  38 
Imbalance,  182 
Inch  system,  41,  42,  43 
Index  of  refraction,  20,  22 
Indirect  method,  99,  100,  155 
Infinity,  35,  68 
Infraduction,  181 
Insufficiencies,  183,  184 
Intensity  of  lids,  9,  10 
Interval,  focal,  123 

of  Sturm,  123 
Inversion,  14 
Iris  in  accommodation,  68 

in  hyperopia,  112 

in  myopia,  118 
Irregular  astigmatism  of  the  cornea, 

125 
of  the  lens,  124 


JACKSON,  209 


K. 


KERATOMETER,  134,  152 
Koratoscope,    134 
Kindergarten  card,  76 


L. 

LENGTH  of  eyeball,  59 

in  emmetropia,  50 
in  hyperopia,  106 
in  myopia,  106 
in  standard  eye,  105 
Lens,  crystalline,  68 
Lenses,  29,  30,  31,  297,  298,  299 
acromatic,  286 
action  of,  31,  32,  33,  35,  36,  37, 

38,  39,  40,  41 
astigmatic,  123 
biconcave,  31,  55 
biconvex,  30,  54 
bifocal,  283,  284.  285,  286,  287 


INDEX. 


Lenses,  collective,  30 

combination,  47,  48.  49,  50,  51, 

52.  53.  54 

crystal,   288 

cylindric,  29,  43,  56,  57 

decentered,  295,  296 

dioptric,  41,  42,  43 

inch,  41,  43 

magnifying,  30 

meniscus,  30 

minifying,  30,  31 

negative,  30,  31 

numeration  of,  41,  42,  43 

perimctric,  394 

periscopic,  30 

planoconcave,  31 

planoconvex,  30 

prismatic,  53,  54 

spheric,  29,  30 

spherocylindric,  45 

tinted,  294 

tone,  290 

trifgcal,  295 

Ligamentum  pectinatum,  66 
Light,  9,  91,  241 

and  shade,  79,  163 

intensity  of,  9,  10 

-screen,  159 

sense,  79 

velocity  of,  9 
Lorgnettes,  289 
Loring,  84,  88 


M. 

MACULA,  59,  85 

Maddox  rod,  188 

Malingerer,  28 

Manifest  refraction,  234,  235 

Meniscus,  30 

Meridians,  132 

Meter,  41,  42 

angle,  83,  84 
Metric  system,  41,  42,  43 
Mires,  152,  153 
Mirror,   14 

concave,  15,  16,  17 

convex,  15,  17,  18 

plane,  14,  158 

reflection  from,  14,  15,  16,  17 
Mixed  astigmatism,  128,  129 
Movements  of  mirror,  161,  162 


Mulatto,  162 

Muscles.    (See  Chapter  VII.) 

ciliary,  66,  67 

Miiller's,  67 

picture  of  (frontispiece). 
Mydriatics,  208 
Myopia,  113 

axial,  113 

causes  of,  115,  116,  117 

description   of,    113,    114,    115. 
248 

diagnosis  of,  nS,  119 

estimation  of,  249 

image  in,  65 

length  of  eyeball  in,  121 

ophthalmoscopic      appearances 
in,   248 

progressive,  242 

symptoms  of,  117,  118 

treatment  of,  238,  239,  240,  249 

N. 

NEAR  point,  So,  228 

determination  of,  80 

Nebula,  265 

Negative  aberration,  173 
angle,  87 

Xerve,  optic,  93 

color  of,  93 

shape  of,  in  astigmatism, 

153 
size  of,  in  hyperopia,  96 

in  myopia,  97 
Nettleship,  121 

Neutralising  lenses,  54,  55,  56,  57,  58 
Nodal  points,  36,  37 
Nystagmus,  157 

O. 

OBSERVER,  91,  92 
Occupation,  224 
Ocular  gymnastics,  203 
Opacities,  125,  265 
Ophthalmometer,  125,  149,  150,  151 
Ophthalmoplegia,  216 
Ophthalmoscope,  88,  113,  119 

how  to  use,  90 

luminous,  101,  102 
Optic  axis,  60,  85 

(enter,  37,  54,  55 

disc,  93 


3I2 


INDEX. 


Optics,  9 
Orbit,  116 
Orthophoria,  177 


P. 

PARALYSIS  of  accommodation,  216 

causes  of,  217 

treatment  of,  218 
Pencil,  converging,  n,  12 

diverging,  12 
Perforated  disc,  141 
Perimeter,  201 
Periodic  squint,  196 
Phenomena  of  light,  12 
Phorometer,  190 
Phorometry,  180 
Pinc-nez,  292 
Pin-hole  disc,  47 
Placido's  disc,  134,  135 
Pointed  line  test,  141 
Points,  cardinal,  59,  60 

nodal,  37 

of  reversal,  163 

principal,   60 
Postcycloplegic,  235,  236 
Pray's  letters,  142 
Presbyopia,  271,  272 

age  of,  272 

causes  of,  272 

description  of,  272,  273 

diagnosis  of,  273 

glasses  for,  274,  275,  276,  277 

symptoms  of,  273 
Prescription  writing,  53,  54 
Principal  axis,  15,  32 

focus,  15,  32 

points,  59,  60 
Prism-diopters,  25,  26 

exercises,  191,  192 

rotary,  189 

Wollaston,  149 
Prisms,  23 

action  of,  23,  24 

centrads,  25 

neutralization  of,  26 

numeration  of,  25 

uses  of,  28,  29 
Punctum  proximum,  69 

determination  of,  79,  So,  81 
in  emmetropia,  69,  70 
in  hyperopia,  72 


Punctum  proximum  in  myopia,  73 
remotum,  68,  69 

determination  of,  72 
in  emmetropia,  69 
in  hyperopia,  71,  72 
in  myopia,  72,  73 
negative,  72 
positive,  73 

Pupil,  size  of,  in  emmetropia,  1 1 
in  hyperopia,  244 
in  myopia,  248 


R. 

RANDALL,  74 

Range  of  accommodation,  69,  70 
in  emmetropia,  70,  71 
in  hyperopia,  71,  72 
in  myopia,  72,  73 
of  convergence,  85 
Rays,  10,  32 

convergent,  n 
divergent,  10,  95 
emergent,  10 
incident,  10 
parallel,  10 
reflected,  10 
refracted,    10 
Reflection,  13 

by  mirrors,  13 
laws  of,  13 
Refraction,  18 

applied.    (See  Chapter  X.) 

by  cylinders,  43,  44 

by  prisms,  23,  24 

by  spheres,  31  to  41  inclusive 

how  to  refract.     (See  Chapter 

IX.) 

index  of,  22 
laws  of,  19 

Regular  astigmatism.      (See   Astig- 
matism.) 
Reisner,  173,  174 
Retina,  69,  94 
Retinal  asthenopia.     (See  Astheno- 

pia.) 

illumination,    162 
image  in  astigmatism,  123 
in  emmetropia,  65 
in  hyperopia,  65 
in  myopia,  65 
reflex,  92 


INDEX. 


3'3 


Retinoscopy.     (See  Chapter  VI.) 
Risley,  117,1 89 
Rods  and  cones,  74 
Rod  test,  1 88 
Room,  91 


S. 

SCHEINER'S  test,  113,  119,  143,  144 
Schematic  eye,  156 
Scissor  movement,  172 
Scopolamin,  213 
Second  sight,  276 
Shadow  test.     (See  Chapter  VI.) 
Shadows  in  retinoscopy,  163 
Simple   hyperopic   astigmatism,    126 

myopic  astigmatism,  i  .-7 
Snellen,  75 
SnoW  blindness,  220 
Spasm  of  accommodation,  218 
causes,  218,  219 
symptoms,  219 
treatment  of,  219 

clonic,  218 

tonic,  218 
Spectacles.    (Sec  Chapter  XII.) 

for  adults,  298 

for  aphakia,  278,  279 

for  astigmatism,  292 

for  children,  298 

for  hyperopia,  295 

for  myopia,  300 

for  presbyopia,   295,   2oS,   2<)<;, 
300 

for  strabismus,  290 

measurements    for,     300,     301 

302,  303,  304,  305,  306 
Spheres.     (See  Lenses.) 
Squint.     (See  Strabismus.) 
Standard  eye,  59 
Static  refraction,  236,  237 
Stenopeic  slit,   135,   136 
Stevens,    182,   189 
Strabismometer,  200 
Strabismus,  195 

alternating,  196 

amount  of,  28,  199,  200,  201 

angle  of,  199 

apparent  angle  of,  199 

causes  of,  196,  197,  198,  K/; 

concomitant,    196 


Strabismus,  constant,  196 

convergent,  195 

divergent,  196 

monolaleral,   196 

paralytic,  196 

periodic,  196 

treatment  of,  28,  201,  202,  203, 

204,  205,  206,  207 
Sturm,  interval  of,  123 
Supraduction,  181 
Surfaces  of  cylinders,  43 

of  mirrors,  14,  15,  16,  17,  18 

of  prisms,  23 

of  spheres,  54,  55 

sursumduction,  181 
Symptoms  of  aphakia,  278,  279 

of  asthenopia,  219,  220 

of  astigmatism,  124 

of  hyperopia,  1 10 

of  myopia,  117,  118 

of  presbyopia,  273 


T. 

TABLE  of  amplitude  of  accommo- 
dation, 70 

of  axial  length  of  eyeball,  121 

of  indexes,  22 

of  lenses,  43 

of  near  points,  272 

of  prisms,  27 
Targets,  152,  153 
Teno'omy,  124,  194,  195,  206,  207 
Test  lor  aphakia,  278,  279 

for  astigmatism,  133 

for  hyperopia,  112,  113 

for  malingering,  28 

for   muscles.          (See    Chapter 
VII.) 

for  myopia,  118,  119 

for  near  point,  Si,  82 

for  vision,  78,  79 

-letters,  74,  75,  76,  77 

-type,  81,  82 
Thomson's  ametrometer,   113,    119, 

148,  149 

Tinted  glasses,  294 
Toric,  290 
Trial-case,  45,  46 
Trial-frame,  47,  48 
Trifocals,  295 


314  INDEX. 

V.  Visual  axis,  60 

VACUUM,  9,  21  normal,  63,  64 

Virtual  focus,  12,  35 

images,  12,  35 
Vision,  acutencss  of,  62,  63,  78,  79  -^r 

binocular,  177 

determination  of,  73,  74,  77,  78       WALLACE,  75 
Visual  acuity,  62,  63,  77,  78,  79  Wollaston,  149 

angle,  61,  62  Worth,  203,  204 


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